A nonsomatic spell is a spell that lacks a somatic component, so it can be cast while wearing full armor and shield, ignoring the chance of arcane spell failure due to armor penalties. This classification is only of interest with regards to arcane spells; divine spells receive no special treatment for being nonsomatic.
The nonsomatic arcane spells included in Neverwinter Nights are blindness/deafness, darkness, displacement, flare, knock, light, mass charm, Mordenkainen's disjunction, polymorph self, power word, kill, power word, stun, time stop, true strike, and wail of the banshee.
In addition, any spell cast with the still spell (or automatic still spell) feat is treated as a nonsomatic spell. Furthermore, epic spells are not considered to be arcane by the game engine, so are not subject to arcane spell failure, even if cast by an arcane caster.