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.

"to synchronize," 1895, from phase, n.. Meaning "to carry out gradually" is from 1949, hence phase in "introduce gradually" (1954), phase out (1954). Related: Phased; phasing.