Glow often refers to the visual effect of air being illuminated near a light source: sun rays through dust, the beam of a flashlight on a foggy night, etc.
In a real-time 3d game, glows are faked using a variety of techniques.
The term "glow map" usually means Emissive map instead. Emissive does not illuminate the air; it does not change the silhouette of a model.
UDK Volumetric Light Beam Tutorial by Epic Games
See also Bloom.