Silence

Silence is a status effect that blocks any sound that would originate from the target. Any spell, both arcane and divine, that requires speech to be cast will fail because of this (that means all spells except for Vocalize, which cures silence). 
It is still possible to cast spells from scrolls, or use wands. Innate abilities, e.g. Bhaalspawn powers like Cure Light Wounds (ability), are not spells and thus not affected by silence. 
Being silenced also prevents a Bard from singing their song, and silenced characters can't speak to others or call for help.
The game engine is hard coded to allow Vocalize (SPWI219) and Teleport to Pocket Plane (SPIN649) to function while under a State: Silenced effect.