A rule in the GameOfChess that allows a pawn to capture a pawn that has moved two spaces by moving into the empty square through which the captured pawn passed, but only in the move immediately following the two-space move.
The term is French for "in passing".
Also, the friendly name for this PerlIdiom:
my $x = "Abc";
(my $y = $x) =~ s/A/D/;
Now, $x is "Abc" and $y "Dbc" ($y was declared, assigned "Abc", and substituted upon)