A wight is an undead creature that is said to resemble a very icky-looking humanoid. They have filthy claws, sparse, thin hair, and their eyes are devoid of pupils (if present at all). They are a fairly powerful form of undead, as they can drain experience levels from other creatures on a successful hit (in AD&D).
In NetHack, the wight is referred to by its more specific name, 'barrow wight' -- this makes sense as wights were supposed to inhabit certain types of graves and/or cemeteries according to old legends. They are significantly less of a threat in NetHack than they are in AD&D. In both systems, wights are affected by holy water and undead turning abilities. In AD&D, curative magic can be used against wights (as with certain other types of undead).