A decoder is a piece of electronic equipment that takes in a signal and changes it into a form another machine can use — often pictures and sound for a television. The box on top of the TV, the unit that unlocks a cable or satellite channel, the chip that turns a coded broadcast into a picture: people call that a decoder. In older living rooms it was the little box you rented from the cable company. The same word also covers other machines that undo a code (a password decoder in a story, a subtitle decoder), but the learner sense here is the home-electronics one: signal in, usable picture and sound out. It is a thing, not usually a person, in this meaning — though decoder can also mean a person who works out a code. Encode is the other direction: put the signal into code. A decoder undoes that. From decode + -er (the thing that decodes).