instantaneous(occurring, arising, or functioning without any delay; happening within an imperceptibly brief period of time) adjective [UK: ˌɪn.stən.ˈteɪ.nɪəs] [US: ˌɪn.stən.ˈtæ.niəs]
momentary(lasting for only a moment) adjective [UK: ˈməʊ.mən.tri] [US: ˈmoʊ.mən.tri]