The algorithm used by a search engine to rank web pages for different search queries. The machine algorithm actually reads a web page's HTML source code and determines the page's relevancy for different search queries. According to that algorithm ranking, the page will be positioned somewhere in a SERP, or not.
The criteria a search engine uses to determine which websites match the words or phrases a visitor is searching for.
A set of rules that a search engine uses to rank the listings contained within its index, in response to a particular query. No search engine reveals exactly how its own algorithm works, to protect itself from competitors and those who wish to spam the search engine.