Short definition
A SERP is the page Google or another search engine returns for a query, including organic links, ads, the map pack, AI Overviews, featured snippets, and other rich features.
In depth
Modern SERPs are no longer ten blue links. A single buyer query can show ads, an AI Overview, a Knowledge Panel, the local map pack, a featured snippet, People Also Ask, video carousels, image results, and finally the organic listings. Different rich features have different click-through rates and require different optimizations to win — AI Overviews want clear answers and schema, the map pack wants reviews and a strong GBP, organic top-three wants topical authority and links. Understanding the SERP shape for a given keyword tells you which optimization to run.
Example
For 'plumber near me', the SERP is dominated by the map pack and AI Overview. A plumber wins by investing in GBP and reviews first, organic second.