That which turns a thing into another form for carrying or storing. Encoding is reversible; that is what separates it from translation.
Latin codex (a book of laws), from caudex (a tree trunk, later a wooden writing block); the agent noun for one who turns a thing into that form.
Its root is the same block of wood the law's own book was once written on, kept exact rather than paraphrased. An encoder is trusted the same way, to hold a form so faithfully that undoing it returns exactly what went in.