A ranking signal or ranking factor is any characteristic of a website that search engine algorithms might consider to calculate its rankings.
Major search engines don't officially disclose the details of their ranking algorithms; however, many sources of information exist that offer insights. These sources include search engine patent filings, extensive testing, case studies, search engine quality guidelines, and statements made by search engine representatives.
Google claims to consider over 200 unique ranking signals to determine how a web page will rank for different keyword searches. Listed below are just a few examples of the more commonly accepted Google ranking factors (some fact, some speculation).
(Search engine ranking signals directly controllable by site owners)
(Influenced by visitor engagement, social media, and other off-site variables)
Note: Ranking signals are not considered a one-size-fits-all, cookie cutter checklist for achieving greater web visibility. Search engines determine website relevance and rankings by looking at the context of signals, and how they naturally fit together on a particular website.