En passant (from French: "in passing") is a maneuver in chess which is performed after a player moves a pawn two squares forward from its starting position, and an opposing pawn captures it as if it had only moved one square. En passant may only be played immediately after a two-square square pawn advance, or the right to capture "in passing" is lost.
After pawns were granted the ability to move two squares on their first move, the En passant rule was introduced in 1490 to prevent pawns from having too much power or freedom.
Here is an example:
In the second diagram, White can NOT capture en passant:
White can not
take here though:
En passant captures are use in chess composition. Many Retrograde analysis problems utilize this fancy captures.
Black could only have moved c7-c5 last move, allowing 1.bxc6