You may play an Action card from your hand that you don't have a copy of in play.
(This is not in the Supply.)
Imp is an Action-Spirit card from Nocturne. It allows you to play an Action from your hand that you don't have a copy of in play; if you have such a card in hand, it acts as a pseudo-lab variant(non-terminal draw), but if you don't, it's only terminal draw. There are 13 copies of Imp in its pile. You can get it via Devil's Workshop, Tormentor, or (like other Spirits) Exorcist.