Feistel network
The construction that lets you build a reversible cipher out of a function that cannot be reversed.
Named after Horst Feistel, the IBM cryptographer who developed it into the Lucifer cipher around 1970. The block is split into two halves. In each round one half is run through a round function with a round key, the result is XORed into the other half, and the halves swap places.
The trick is that the round function need not be invertible — decryption is the same network with the round keys in reverse order. That made it possible to build secure ciphers out of arbitrarily messy functions. DES, Blowfish and GOST are Feistel-based. AES is not; it uses a substitution-permutation network in which the whole block is processed every round.
Sources
NIST FIPS 46-3