Sometimes the reason a piece should not be captured isn’t the recapture, but rather the threat of a fork because the capturing piece would find itself in just the wrong spot. The 1926 game between Stepanov and Romanovsky is full of this after 20. … R×f3+.
Is there a name for the situation where a piece is protected by the threat of a fork?
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.
I don't know about English. In spanish we use the term "defensa indirecta" or "protección indirecta", but it does not refer specifically to indirect protection/defence by fork, it could be a threat of any kind that prevents the piece in question to be captured.
For example, after the silly moves 1.e4 d6 2.Nf3 Bg4 3. Bc4 g6 4.Ne5 and the queen is indirectly protected by a mate threat.
After 1.e4 e6 2.d4 d5 3.e5 c5 4.c3 Nc6 5.Nf3 Qb6 6.Bd3, the d4 pawn is indirectly protected by a discovered attack in case it got captured.
And finally, relating back to your specific case: 1.d4 c5 2.dxc5 e6 3.Be3 Qa5+ 4.c3 is another case of indirect protection, this time by means of a fork threat in case of 4...Bxc5?
