Rainbow Table

A rainbow table↗ is a large precomputed table that maps common passwords to their corresponding hash values, allowing an attacker who has stolen a database of password hashes to look up a hash and instantly find the matching plaintext password, rather than having to guess and hash each possibility one at a time.

Rainbow tables make cracking unsalted password hashes dramatically faster. The standard defense is salting↗, adding a unique random value to each password before hashing↗ it, which makes precomputed tables useless since the same password now produces a different hash for every single user.