The Collaborative International Dictionary of English v.0.48:

Mutism \Mut"ism\, n.
   The condition, state, or habit of being mute, or without
   speech. --Max M["u]ller.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mutism
    n 1: the condition of being unable or unwilling to speak; "her
         muteness was a consequence of her deafness" [syn: mutism,
         muteness]