That which turns an encoded form back into what it carried. Decoding recovers; it does not interpret.
Latin de- (reversal) + codex (a system of symbols, from caudex, a wooden writing tablet); that which turns an encoded form back into what it carried, recovering rather than interpreting.
Its root, codex, is a wooden tablet a code was once written on, and a decoder still only reverses that writing, never adds to it. What comes out the other side is exactly what went in, restored rather than explained.