The share of people who completed a specific action (clicking, signing up, paying) out of everyone who had the chance to do it.
Amazon filed US Patent 5,960,411 in 1997 covering single-click ordering, and granted in 1999. Internal estimates at the time pegged the conversion lift at roughly 5%, which at Amazon's scale translated into hundreds of millions in incremental revenue annually. The patent was so valuable Amazon licensed it to Apple in 2000 for use on iTunes.