The Collaborative International Dictionary of English v.0.48:

Insomnia \In*som"ni*a\, n. [L., fr. insomnis sleepless; pref.
   in- not + somnus sleep.]
   Lack of sleep; inability to sleep, especially when chronic;
   wakefulness; sleeplessness.
   [1913 Webster]

WordNet (r) 3.0 (2006):

insomnia
    n 1: an inability to sleep; chronic sleeplessness [ant:
         hypersomnia]