Search engine optimization (SEO)
Serch engine optimization is the practice of improving the visibility and overall performance of websites and web pages in search engine results pages (SERPs).[1] It focuses on increasing the quantity and quality of traffic from unpaid (organic) search results rather than paid advertising.[2] SEO applies to multiple search formats, including web, image, video, news, academic, and vertical search engines, as well as AI-assisted search interfaces.
SEO is commonly used as part of a broader digital marketing strategy and involves optimizing technical infrastructure, content relevance, and authority signals to improve rankings for user queries.[3] The objective of SEO is to attract users who are actively searching for information, products, or services, thereby improving brand visibility, user engagement, and conversions.[4]
History
Webmasters and content providers began optimizing websites for search engines in the mid-1990s as the first search engines were cataloging the early Web. Search engine users would query the URL of a page, and then receive information found on the page, if it existed in the search engine’s index.
ALIWEB and the earliest versions of search engines required website developers to manually upload website index files in order to be searchable and widely did not utilize any form of ranking algorithm for user queries.[1] The emergence of automated web crawlers would later be used to proactively discover and index websites. This led to website developers to optimize their website’s search signals, including the use of meta tags, to achieve greater visibility in search results.
According to a 2004 article by former industry analyst and current Google employee Danny Sullivan, the phrase “search engine optimization” came into use in 1997. Sullivan credits SEO practitioner Bruce Clay as one of the first people to popularize the term.[5]
In some cases, early search algorithms weighted particular HTML attributes in ways that could be leveraged by web content providers to manipulate their search rankings.[6] As early as 1997, search engine providers began adjusting their algorithms to prevent these actions.[3] Eventually, search engines would incorporate more meaningful measures of page purpose, including the more recent development of semantic search.[7]
Some search engines frequently sponsor SEO conferences, webchats, and seminars. Major search engines provide information and guidelines to help with website optimization.[8][4] Google has a Sitemaps program to help webmasters learn if Google is having any problems indexing their website and also provides data on Google traffic to the website.[2] Bing Webmaster Tools provides a way for webmasters to submit a sitemap and web feeds, allows users to determine the “crawl rate”, and track the web pages index status.
In 2015, it was reported that Google was developing and promoting mobile search as a key feature within future products, resulting in brands and marketers shifting toward mobile-first experiences.[9]
Relationship between SEO and large language models
In the 2020s in response to the AI boom, digital marketers developed new optimization approaches for LLM-based search, referred to as answer engine optimization (AEO) or generative engine optimization (GEO).[10][11] These terms are often used interchangeably,[12][13] and other related terms exist, such as AI engine optimization (AIEO).[14]
Digital marketers optimizing for GEO tend to focus on content structure, authority signals, and structured data, with the goal of becoming a trusted, citable source in AI-generated responses.[15][11]
However, Google states that its AI Overviews and AI Mode do not require additional technical optimization beyond the existing requirements for appearing in Google Search. Pages must be indexed and eligible to appear in Search, while established SEO practices such as allowing crawling, using internal links, providing accessible textual content, and maintaining useful, reliable, people-first content remain applicable to these AI features.[16]
Relationship between Google and SEO industry
In 1998, two graduate students at Stanford University, Larry Page and Sergey Brin, developed “Backrub”, a search engine that relied on a mathematical algorithm to rate the prominence of web pages. The number calculated by the algorithm, PageRank, is a function of the quantity and strength of inbound links.[17] PageRank estimates the likelihood that a given page will be reached by a web user who randomly surfs the web and follows links from one page to another. In effect, this means that some links are stronger than others, as a higher PageRank page is more likely to be reached by the random web surfer.
Page and Brin founded Google in 1998.[18] Google attracted a loyal following among the growing number of Internet users, who liked its simple design.[19] Off-page factors (such as PageRank and hyperlink analysis) were considered as well as on-page factors (such as keyword frequency, meta tags, headings, links and site structure) to enable Google to avoid the kind of manipulation seen in search engines that only considered on-page factors for their rankings. Although PageRank was more difficult to game, webmasters had already developed link-building tools and schemes to influence the Inktomi search engine, and these methods proved similarly applicable to gaming PageRank. Many sites focus on exchanging, buying, and selling links, often on a massive scale. Some of these schemes involved the creation of thousands of sites for the sole purpose of link spamming.[20]
By 2004, search engines had incorporated a wide range of undisclosed factors in their ranking algorithms to reduce the impact of link manipulation.[21] The leading search engines, Google, Bing, and Yahoo, do not disclose the algorithms they use to rank pages. Some SEO practitioners have studied different approaches to search engine optimization and have shared their personal opinions.[22] Patents related to search engines can provide information to better understand search engines.[23] In 2005, Google began personalizing search results for each user. Depending on their history of previous searches, Google crafted results for logged in users.[24]
In 2007, Google announced a campaign against paid links that transfer PageRank.[25] On June 15, 2009, Google disclosed that they had taken measures to mitigate the effects of PageRank sculpting by use of the nofollow attribute on links. Matt Cutts, a well-known software engineer at Google, announced that Google Bot would no longer treat any no follow links, in the same way, to prevent SEO service providers from using no follow for PageRank sculpting.[26] As a result of this change, the usage of no follow led to evaporation of PageRank. In order to avoid the above, SEO engineers developed alternative techniques that replace no followed tags with obfuscated JavaScript and thus permit PageRank sculpting. Additionally, several solutions have been suggested that include the usage of iframes, Flash, and JavaScript.[27] As a response, in 2019, Google would change its position and started using these tags as hints as a way to better understand how to appropriately analyze and use links within their systems”.[28]
In December 2009, Google announced it would be using the web search history of all its users in order to populate search results.[29] On June 8, 2010 a new web indexing system called Google Caffeine was announced. Designed to allow users to find news results, forum posts, and other content much sooner after publishing than before, Google Caffeine was a change to the way Google updated its index in order to make things show up quicker on Google than before. According to Carrie Grimes, the software engineer who announced Caffeine for Google, “Caffeine provides 50 percent fresher results for web searches than our last index…”[30] Google Instant, a real-time search feature, was introduced in late 2010 to deliver more timely and relevant results. Historically, site administrators often spent months or years optimizing websites to improve their search rankings. With the rise of social media platforms and blogs, major search engines adjusted their algorithms to enable fresh content to rank more quickly in search results.[31]
Google has implemented numerous algorithm updates to improve search quality, including Panda (2011) for content quality, Penguin (2012) for link spam, Hummingbird (2013) for natural language processing, and BERT (2019) for query understanding. These updates reflect the ongoing evolution of search technology and Google’s efforts to combat spam while improving user experience.
Legal disclosures during the United States v. Google antitrust trial, which began in 2020, as well as the 2024 Google Search documentation leak, revealed more details on how Google’s SEO system operated, including the usage of click-based metrics and Chrome browser data, which Google had previously denied.
On May 20, 2025, Google announced that AI Mode would be released to all U.S. users. AI Mode uses what Google calls a “query fan-out technique” which breaks down the search query into multiple sub-topics which generates additional search queries for the user.[32]
Methods
Getting indexed
The leading search engines, such as Google, Bing and Yahoo!, use crawlers to find pages for their algorithmic search results. Pages that are linked from other search engine-indexed pages do not need to be submitted because they are found automatically. The Yahoo! Directory and DMOZ, two major directories which closed in 2014 and 2017 respectively, both required manual submission and human editorial review.[33] Google offers Google Search Console, for which an XML Sitemap feed can be created and submitted for free to ensure that all pages are found, especially pages that are not discoverable by automatically following links[34] in addition to their URL submission console.[35] Yahoo! formerly operated a paid submission service that guaranteed to crawl for a cost per click;[36] however, this practice was discontinued in 2009. Nevertheless, SEO tools such as Semrush enable analysis of both paid and organic traffic by providing insights into cost per click and keyword performance.[37]
Search engine crawlers may look at a number of different factors when crawling a site. Not every page is indexed by search engines. The distance of pages from the root directory of a site may also be a factor in whether or not pages get crawled.[38]
Mobile devices are used for the majority of Google searches.[39] In November 2016, Google announced a major change to the way they are crawling websites and started to make their index mobile-first, which means the mobile version of a given website becomes the starting point for what Google includes in their index.[40]
Preventing crawling
To avoid undesirable content in the search indexes, webmasters can instruct spiders not to crawl certain files or directories through the standard robots.txt file in the root directory of the domain. Additionally, a page can be explicitly excluded from a search engine’s database by using a meta tag specific to robots (usually <meta name=”robots” content=”noindex”> ). When a search engine visits a site, the robots.txt located in the root directory is the first file crawled. The robots.txt file is then parsed and will instruct the robot as to which pages are not to be crawled. As a search engine crawler may keep a cached copy of this file, it may on occasion crawl pages a webmaster does not wish to crawl. Pages typically prevented from being crawled include login-specific pages such as shopping carts and user-specific content such as search results from internal searches. In March 2007, Google warned webmasters that they should prevent indexing of internal search results because those pages are considered search spam.[41]
In 2020, Google sunsetted the standard (and open-sourced their code) and now treats it as a hint rather than a directive. To adequately ensure that pages are not indexed, a page-level robot’s meta tag should be included.[42]
Increasing prominence
A variety of methods can increase the prominence of a webpage within the search results. Cross linking between pages of the same website to provide more links to important pages may improve its visibility. Page design makes users trust a site and want to stay once they find it. When people bounce off a site, it counts against the site and affects its credibility.[43]
Writing content that includes frequently searched keyword phrases so as to be relevant to a wide variety of search queries will tend to increase traffic. Updating content so as to keep search engines crawling back frequently can give additional weight to a site. Adding relevant keywords to a web page’s metadata, including the title tag and meta description, will tend to improve the relevancy of a site’s search listings, thus increasing traffic. URL canonicalization of web pages accessible via multiple URLs, using the canonical link element[44] or via 301 redirects can help make sure links to different versions of the URL all count towards the page’s link popularity score. These are known as incoming links, which point to the URL and can count towards the page link’s popularity score, impacting the credibility of a website.[43]