Short definition
GEO (Generative Engine Optimization) is the practice of structuring a website so that AI engines like ChatGPT, Perplexity, Claude, and Google AI Overviews cite it as a source.
In depth
GEO is the AI-era counterpart to classic SEO. Where SEO targeted blue links and rankings, GEO targets citations inside generative answers. The tactics overlap with SEO (clean HTML, schema, fresh content, authority) but add new ones: direct-answer paragraphs in the first 60 words of a page, llms.txt and llms-full.txt files, allowing AI crawlers in robots.txt, comparison and definition content (the formats LLMs cite most), structured data on every entity, and original statistics or quotes the model can attribute back to your URL.
Example
A web agency adds an /llms.txt file, switches every article to lead with a direct-answer paragraph, and within a quarter starts appearing as a cited source in ChatGPT answers about 'web design pricing'.