means a cryptanalysis method where the attacker knows some original unencrypted text and its encrypted version, then uses that pair to figure out the key or algorithm.

from formalized in classical cryptanalysis as one of several attack models ranked by how much the attacker knows; the term crystallized during wwii-era codebreaking, when known phrases like weather reports or standard sign-offs in enemy messages gave analysts a toehold into otherwise scrambled ciphers.