en passant

Sweetcheeks provided a good description.
Another way to memorize it is to understand logic behind why it exists. The special movement a pawn has on its starting square cannot be used to avoid the opportunity for a player to capture with their pawn, so a player can still opt to move their pawn the two squares but then the other player is given the right (for their next move only) to capture it with their own pawn as if it had only moved one square.