The complete, sourced definition of search engine optimisation — what it is, how it works, and what it means in 2026.
SEO (Search Engine Optimisation) is the practice of improving a website's visibility in unpaid (organic) search engine results by making the content, structure, and authority of the site more relevant and trustworthy to search engines and the people using them.
Google's own framing: "SEO — or Search Engine Optimization — is the practice of making your website more visible in organic search results." — Google Search Central documentation.
Every SEO activity falls into one of three areas:
Search engines use automated programs called crawlers (or spiders) to discover and read web pages. Google's crawler, Googlebot, follows links from page to page, downloading content and storing it in a large database called the index. When a user performs a search, Google's ranking algorithm retrieves relevant pages from the index and orders them by relevance and quality.
The ranking algorithm considers hundreds of factors. The most consistently documented and significant include: the relevance of the page content to the search query, the quality and quantity of backlinks pointing to the page, the page's technical performance (speed, mobile usability), and signals of expertise, authoritativeness, and trustworthiness (E-E-A-T).
E-E-A-T stands for Experience, Expertise, Authoritativeness, and Trustworthiness. It is Google's framework for evaluating content quality, documented in Google's Search Quality Evaluator Guidelines. It matters most for YMYL (Your Money or Your Life) topics — health, finance, legal, and safety content — where the quality of information has real-world consequences.
SEO produces organic (unpaid) rankings. Search Engine Marketing (SEM) / Google Ads produces paid rankings. The two appear on the same search results page but operate independently. Organic rankings take time to build but do not incur a per-click cost. Paid rankings appear immediately but stop when the budget does. Most mature marketing strategies use both.
AI has affected SEO in two directions simultaneously. On the search side, Google's AI systems (BERT, MUM, the Helpful Content system) have made the algorithm much better at understanding meaning rather than keyword matching, and at detecting content created primarily to rank rather than to help users. On the content creation side, AI writing tools have flooded the internet with low-quality content, making Google's quality signals more important than ever. High-quality, human-expert-led content that genuinely answers questions has increased in value as generic content has decreased in effectiveness.