Different openings

Originally posted by Derfel Cadarn
If someone plays 1.d4 and you respond 1...c5 is it still the Sicilian Defense?

No. 1d4 c5 is the Benoni Defense. The Sicilian is a defense against 1. e4.

EDIT- Unless white follows up with 2. e4. Then you have a Sicilian Smith-Morra Gambit by transposition.

Originally posted by Derfel Cadarn
If someone plays 1.d4 and you respond 1...c5 is it still the Sicilian Defense?

No, until a few more moves have been made, it's not anything.

If White plays 2.d5, the game will probably develop into a Queen's Pawn opening, such as the Benoni, although it could transpose into a Pirc Defence, which is a King's Pawn opening.

If White responds with 2.dxc5, it becomes a Sicilian, Pirc, Modern Defence or just as easily beome a hybrid set-up, that is hard to classify as a particular opening.

White could also play a waiting move like 2.e3, which keeps his options open, although it probably rules out the possibility of transposing to a Sicilian, as he won't want to play e4 at a later stage and waste a tempo.