This term is used as a
more-or-less informal measure of the power of a
weapon and is generally used in the context of
small arms. It does not necessarily refer to whether or not the weapon can
kill someone, but rather the weapon's
ability to
incapacitate or otherwise render
immobile the target. Generally, the idea is that a weapon with good stopping power, when fired at an
oncoming attacker, will
knock his ass on the ground.
Stopping power is derived from the kinetic energy the weapon is able to impart to the target. For instance, a 9-mm pistol fires small bullets at a relatively low speed. It is about as effective as a paintball gun against, say, a couple of bank robbers in full body armor. On the opposite end of the spectrum, an M-16 fires relatively large caliber bullets at a very high speed. If these don't succeed in penetration, they still have enough kinetic energy to put the guy down on the ground, giving you a tremendous advantage in whatever it is you're trying to do, like reading his gas meter or selling him encyclopedias.