A ranking factor is any signal a search engine's algorithm weighs when deciding how to order results for a query. Google has said its algorithm considers well over 200 such signals, though it's never published a complete, definitive list, which is exactly why so much SEO folklore and speculation has built up around the topic over the years.
Some factors are confirmed directly by Google: page experience and Core Web Vitals, mobile-friendliness, HTTPS, quality backlinks, and genuinely relevant, well-written content are all things Google has spoken to explicitly. Others are widely believed based on correlation studies and practitioner testing but never officially confirmed, and a fair number of "ranking factors" circulated in SEO content are outdated, overstated, or outright myths (word count and keyword density are common examples). The most reliable approach is treating officially confirmed factors as the foundation, staying skeptical of any single factor presented as a silver bullet, and remembering that Google's actual algorithm weighs hundreds of signals together rather than any one of them in isolation.
