Reflector
The fixed disc in the Enigma that sent the signal back through the rotors and made encryption self-reciprocal.
The reflector pairs up the 26 letters, so every letter has exactly one partner. The signal travels through the rotors, bounces off the reflector and returns through them along a different path. That is why the same setting both encrypts and decrypts. It also means no letter can ever encrypt to itself, a property Bletchley Park exploited when testing guessed words, or cribs, against intercepted traffic. It is the only rotor that does not turn, and arguably the most underrated part of the machine.