A gambit (from ancient Italian gambetto, meaning "to trip") is a chess opening in which a player, more often White, sacrifices material, usually a pawn, with the hope of achieving a resulting advantageous position. Some well-known examples are the King's Gambit (1.e4 e5 2.f4), Queen's Gambit (1.d4 d5 2.c4), and Evans Gambit (1.e4 e5 2.Nf3 Nc6 3.Bc4 Bc5 4.b4). A gambit used by Black may also be called a gambit (e.g. the Latvian Gambit - 1.e4 e5 2.Nf3 f5 or Englund Gambit - 1.d4 e5), but is sometimes called a "countergambit" (e.g. the Albin Countergambit - 1.d4 d5 2.c4 e5 and Greco Counter-Gambit, an old-fashioned name for the Latvian Gambit).
The word "gambit" was originally applied to chess openings in 1561 by Spanish priest Ruy López de Segura, from an Italian expression dare il gambetto (to put a leg forward in order to trip someone). Lopez studied this maneuver, and so the Italian word gained the Spanish form gambito that led to French gambit, which has influenced the English spelling of the word. The broader sense of "opening move meant to gain advantage" was first recorded in English in 1855.
Gambits are often said to be 'offered' to an opponent, and that offer is then said to be either 'accepted' or 'declined.' If a player who is offered a gambit captures the piece (and thus gains material) the gambit is said to be accepted. If the player who was offered the gambit ignores it and instead continues his or her development, then the gambit is said to be declined.
In modern chess, the typical response to a moderately sound gambit is to accept the material and give the material back at an advantageous time. For gambits that are less sound, the accepting player is more likely to try to hold on to his extra material. A rule of thumb often found in various primers on chess suggests that a player should get three moves (see tempo) of development for a sacrificed pawn, but it is unclear how useful this general maxim is since the "free moves" part of the compensation is almost never the entirety of what the gambiteer gains. Of course, a player is not obliged to accept a gambit. Often, a gambit can be declined without disadvantage.
A gambit is said to be 'sound' if it is capable of procuring adequate concessions from the opponent. There are three general criteria in which a gambit is often said to be sound:
A good example of a sound gambit is the Scotch Gambit: 1.e4 e5 2.Nf3 Nc6 3.d4 exd4 4.Bc4. Here Black can force White to sacrifice a pawn speculatively with 4...Bb4+, but White gets very good compensation for one pawn after 5.c3 dxc3 6.bxc3, or for two pawns after 6.0-0 inviting 6...cxb2 7.Bxb2, due to the development advantage and attacking chances against the black king. As a result, Black is often advised not to try to hold on to the extra pawn. A more dubious gambit is the so-called Halloween Gambit: 1.e4 e5 2.Nf3 Nc6 3.Nc3 Nf6 4.Nxe5?! Nxe5 5.d4. Here the investment (a knight for just one pawn) is too large for the moderate advantage of having a strong center.
Read main article: List of chess gambits