En passant
A pawn that advances two squares from its starting rank may be captured by an enemy pawn as if it had stopped on the intermediate square. The right exists only on the immediately following move — after that the window closes forever. In notation it looks like an ordinary pawn capture, for example exd6.
Black has just played d7–d5; White may reply exd6 e.p., removing the d5-pawn — but only right now.