Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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