ALCM stands for Air-Launched
Cruise Missile. It is both a generic and a specific; in the generic, it refers to any weapon which is employed in this manner. In the specific, it refers to a particular weapon system built and deployed by the United States Air Force.
These weapons were designed to be dropped from a B-52 Stratofortress nuclear bomber in the event of nuclear war to then fly deeper into Soviet airspace towards their targets. They were intended to increase the survivability and likelihood of success of the American bomber force by allowing them to launch their attacks from outside Soviet airspace proper. The ALCM was built by Boeing and consists of a small, pilotless jet aircraft with a guidance system and (originally) a nuclear warhead in the nose. Upon being dropped from the launch vehicle, the ALCM unfolds small wings, ignites its small turbojet and heads off towards its rendevous with apocalypse. The ALCM is designed to fly low, following terrain, in order to avoid detection and interdiction by air defenses.
The U.S. Air Force opened Gulf War I with a large strike on Iraq which included a massive salvo of ALCMs that had been converted to carry conventional high explosives. It seems likely that there is a pressure to 'use them or lose them' since as they age and are removed from the inventory no replacements are to be had.
The original ALCM was designated (as deployed) the AGM-86B (for 'Air-to-Ground Missile') and carried a nuclear payload. Those converted to a conventional role were redesignated the CALCM (yep, Conventional ALCM) and given the designation AGM-86C. A 3,000-lb blast/fragmentation warhead was fitted, and the TERCOM guidance system updated to use GPS. A later variant, the AGM-86D, sports a penetrating warhead to enable it to attack hardened targets.
A total of 1,739 missiles were produced between 1980 and 1985. Some of these were later converted to the CALCM models; no new production was made for CALCM stocks.