(n)an emotional episode (usually in young children) in which the person awakens in terror with feelings of anxiety and fear but is unable to remember any incident that might have provoked those feelings
The Collaborative International Dictionary of English (GCIDE) v.0.53Collaborative International Dictionary (GCIDE)
. (Med.) A sudden awakening associated with a sensation of terror, occurring in children, esp. those of unstable nervous constitution. [ Webster 1913 Suppl. ]