In chess, a half-open file (or semi-open file) is a file with only pawns of one color. The half-open file can provide a line of attack for a player's rook or queen. A half-open file is exploited by the player with no pawns on it.
Many openings, such as the Sicilian Defense, aim to complicate the position. In the main line Sicilian, 1.e4 c5 2.Nf3 d6 (or 2...e6, or 2...Nc6) 3.d4 cxd4 4.Nxd4, White obtains a half-open d-file, but Black can pressure White along the half-open c-file.
The game Loek van Wely-Judit Polgár, Hoogeveen, 1997 demonstrates the power of half-open files in attacks. Despite having one fewer pawn than White, Black's possession of two powerful half-open files (her rook on the f-file and queen on the g-file) gives her a winning advantage.
Black played 30...Rxf2+! and White resigned, anticipating 31.Rxf2 Qxg3+ 32.Kf1 Qxf2#.