Entry 1 sense Webster, 1913 Insomnia /ĭnˌ-säm'-nē-ə/ · In·som·ni·a · IPA /ɪnˈsɒmniə/ 01 n. Lack of sleep; inability to sleep, especially when chronic; wakefulness; sleeplessness. 1. Lack of sleep; inability to sleep, especially when chronic; wakefulness; sleeplessness.