A SERP is an accronym for Search Engine Results Pages. A SERP is the listing of web pages returned by a search engine in response to a keyword query. The results normally include a list of web pages with titles, a link to the page, and a short description (snippet) showing where the keywords have matched content within the page. A SERP may refer to a single page of links returned, or to the set of all links returned for a search query.
Some Search Engines (SE) cache their SERPs for frequent searches and display the cached SERP instead of a live SERP to increase the performance of the search engine. The search engine updates the SERPs periodically to account for new pages, and possibly to modify the rankings of pages in the SERP.
SERP refreshing can take several days or weeks which can occasionally cause results to be inaccurate or out of date, and new sites and pages to be completely absent.