Perceptual Hashes

Definition

A perceptual hash differs from a cryptographic hash in that it varies only slightly with slight differences in source files, whereas a cryptographic hash produces a completely different output number even if a single bit of the source file is changed. This allows for a measurement of the amount of difference between files in order to determine how similar one piece of content is with another, ergo how derivative it might be, what parts might have been copied from another work that might be copyrighted.