Mexican standoff refers to a situation where two-plus people have guns pointed at each other and they're all afraid of moving, what with the weaponry and all. Found in the film The Mexican, and also the following films (first four featuring Quentin Tarantino, it being a trademark of his):

Reservoir Dogs: between Joe, Mr. White and Eddie
True Romance: between Lee's men, cops and mafia
Natural Born Killers: between cops, Mickey and Scagnetti
Pulp Fiction: Between Jules, Vincent, Honeybunny and Pumpkin
Face/Off: between John Travolta and Nicholas Cage's characters
The Good, The Bad, and the Ugly
The Wild Bunch: the namesake and a gang of Mexicans.
The Matrix: between Neo and Agent Smith.
The Matrix Revolutions: between Trinity, the Merovingian, and the Merovingian's thugs.