Short definition
GEO (Generative Engine Optimization) is the practice of structuring a website so automated answer systems can understand the source, claims, and entities on the page.
In depth
Generative Engine Optimization is an emerging search discipline focused on making pages clear, source-backed, and easy for automated answer systems to understand. The tactics overlap with SEO: clean HTML, accurate schema, fresh content, topical authority, visible authorship, and original statistics or quotes that can be checked.
Example
A web agency rewrites a pricing guide so the answer, assumptions, examples, and source notes are clear to both buyers and automated systems.