I agree with asdf regarding the idea that there isn't a name specifically attached to indirect protection of a piece stemming from a fork.
However, in general, pieces that are protected by any tactic, including forking, are said to be "indirectly defended."
As far as a name for the piece being indirectly defended, one might call it a "bait" piece, as it baits the opposing player to make a dubious move. In a specific opening line, there is a somewhat famous "poisoned pawn" - a pawn that looks like it can be taken without fear, but makes things positionally difficult in the long run. Thus, an indirectly defended piece might also be called a "poisoned" piece.
Edit: as Cyriac Antony points out, this is also known as an "indirectly defended piece."