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

My definition? The inability to sleep when you want to, accompanied by the inability to stay awake when you want to. Being buzzed by mosquitoes at 1 a.m. doesn't help.