En passant (French for “in passing”) is a special pawn capture in chess. When a pawn advances two squares from its starting square and lands beside an opponent’s pawn on the same rank, that opponent’s pawn may capture it as if it had moved only one square, landing on the square the pawn passed over. Under FIDE’s Laws of Chess, the capture is legal only on the very next move; if the player does not take it immediately, the right is lost. In notation it is written like a normal pawn capture, optionally followed by “e.p.” (for example, exd6 e.p.).
Source: FIDE Laws of Chess, Article 3.7.3
Image: Staunton chess set in the starting position (photo by Wilfredor, CC0), via Wikimedia Commons.