1705, "phase of the moon," back-formed as a singular from Modern Latin phases, plural of phasis, from Greek phasis "appearance" (of a star), "phase" (of the moon), from stem of phainein "to show, to make appear" (see phantasm). Latin singular phasis was used in English from 1660. Non-lunar application is first attested 1841. Meaning "temporary difficult period" (especially of adolescents) is attested from 1913.[phase etymology, phase origin, 英语词源]