PageRank
Google's original algorithm that measures page importance based on the quantity and quality of backlinks.
PageRank is the algorithm Google was founded on, created by Larry Page and Sergey Brin at Stanford in 1996. It measures the importance of web pages based on the links pointing to them. The idea: a link from one page to another is a "vote," and pages with more votes from important pages are themselves more important.
Google stopped publicly showing PageRank scores in 2016 (it used to be a toolbar metric from 0-10), but the underlying algorithm is still part of Google's ranking system, though it's been heavily modified and supplemented with hundreds of other signals.
The math behind PageRank is elegant: a page's rank is determined by the rank of all pages linking to it, divided by the number of outgoing links on each of those pages. This means one link from a high-authority page can be worth more than thousands of links from low-quality pages.
While we can't see PageRank directly anymore, third-party metrics like Domain Authority (Moz) and Domain Rating (Ahrefs) attempt to approximate similar concepts.
Why It Matters for SEO
PageRank was the innovation that made Google better than every other search engine. Understanding how link equity flows between pages helps you build better internal linking strategies, evaluate backlink quality, and understand why some pages rank better than others.