halo

HALO (all caps) stands for High Altitude Low Opening. (See also PJ, the pipeline for more info.)

"HALO stands for High Altitude Low Opening; it's used to drop PJs into hot areas where a more leisurely deployment would get them all killed. In terms of violating the constraints of the physical world, HALO jumping is one of the more outlandish things human beings have ever done. The PJs jump from so high up—as high as 40,000 feet—that they need bottled oxygen to breathe. They leave the aircraft with two oxygen bottles strapped to their sides, a parachute on their back, a reserve 'chute on their chest, a full medical pack on their thighs, and an M-16 on their harness. They're at the top of the troposphere—the layer where weather happens—and all they can hear is the scream of their own velocity. They're so high up that they freefall for two or three minutes and pull their 'chutes at a thousand feet or less. That way, they're almost impossible to kill."
—Sebastian Junger, The Perfect Storm, 1997 (NY: HarperCollins, 1999), 177