Gambit
A gambit offers material — usually a pawn, sometimes more — in the opening to buy development time, open lines or seize the centre. The defender can often return the material at the right moment and consolidate, so a gambit is best understood as an investment rather than a gift. The Queen's Gambit is the most famous example, though its pawn can usually be regained.
The Queen's Gambit: 2.c4 offers the c-pawn to pull the d5-pawn away from the centre.