The King's Gambit is the opening that begins with the moves 1. e4 e5 2. f4, an aggressive sacrifice less popular today than in chess' romantic attacking era.
The King's Gambit runs as follows:
[FEN ""] 1. e4 e5 2. f4 ...
This opening is much less popular today than it was in the attack-happy Romantic era of chess, but it still appears occasionally at the highest levels of the game. Hikaru Nakamura, for example, who is in the world's top 10 as of this writing, sometimes plays the King's Gambit against elite players, e.g. Nakamura-Adams (2011).