An Etymological Dictionary of Astronomy and Astrophysics

1. A particular stage or point in a course, development,
   or graph varying cyclically; the fractional part of the period through which the time has advanced, measured from some arbitrary origin. Phase is measured like an angle, when a complete cycle is equivalent to a phase of 360° (or 2π radians), or, sometimes, as a number between 0 and 1. Two or more waves of the same frequency are → in phase when their maxima and minima take place at the same moments. Otherwise, they are said to be → out of phase or that they have a → phase difference.
   

2. A state in which matter can exist, depending on temperature and pressure, e.g. the → solid, → liquid, → gaseous, and → plasma states.
   

3. A recurring form of the → Moon or a → planet seen in the sky. → lunar phase, → phases of Venus.
   

4. In a → binary star system, → orbital phase.

Etymology (EN): Mod.L. phases, plural of phasis, from Gk. phasis “appearance,” from stem of phainein “to show, to make appear.”

Etymology (PE): 1) Fâz, loanword from Fr., as above.

2. Simâ “face, aspect, resemblance.”