A Pawn (often White's Pawn on b2) which is undefended during the opening but which if taken, often permits the player who gave up the Pawn to engage in a strong attack or to later win the piece taking the Pawn.
An unprotected pawn which, if captured, causes positional problems or material loss. It is also a variation of the Sicilian Defense, where some players call White's pawn on b2 a poisoned pawn.