From CryptoDox, The Online Encyclopedia on Cryptography and Information Security
A reciprocal cipher is a shared-key cryptographic system in which there is no difference between "encoding" and "decoding". If a person "encrypts" some ciphertext (with the same key) with the system, cleartext comes out. (In non-reciprocal ciphers, repeatedly encrypting does *not* result in the original plaintext).
Some famous reciprocal ciphers include
Most mechanical cipher machines use a reciprocal cipher, so it wouldn't need a seperate "encode mode" and "decode mode".