Civilian casualties in a war zone, sometimes in large numbers. Much more antiseptic-sounding than "we murdered a bunch of innocent people to achieve these military objectives". The phrase was popularized during the Gulf War (as seen on CNN); first it was used in military briefings, then it spread like wildfire to all the anchors and reporters covering the war. Saves time and print-space, since "murdered a bunch of innocent people to achieve these military objectives" is quite the mouthful.