/ ˌɪnstənˈteɪniəs; NAmE / adjective
happening immediately :
an instantaneous response
Death was almost instantaneous.
► in·stant·an·eous·ly adverb
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WORD ORIGIN
mid 17th cent.: from medieval Latin instantaneus , from Latin instant- being at hand (from the verb instare ), on the pattern of ecclesiastical Latin momentaneus .