This article is from the Magic: The Gathering Rules FAQ, by Patrik Linell (email@example.com) with numerous contributions by others.
A "creature" is a term reserved for a permanent. In other words,
the only way something can be a "creature" is for it to be in play.
Anywhere else, the thing is just a "creature card" (or "creature spell", if
it's on the stack). The same applies to the terms "land", "artifact" and
So Unsummon, which says "Return target creature to its owner's hand",
can't be used to retrieve creature cards from your graveyard, because
they're not creatures.