friendly fire

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Discharge of a military weapon that injures or kills a member of one's own armed forces or an ally.

Wiktionary

1. n. Weapons fire from allied or friendly forces, as opposed to fire coming from enemy forces or enemy fire.

GNU Webster's 1913

1. n. Weapons fire{9}, such as artillery or aerial bombardment, from one's own forces; -- used mostly when troops do damage or cause casualties among their own forces.

WordNet 3.0

1. n. fire that injures or kills an ally