An Etymological Dictionary of Astronomy and Astrophysics

Something that may or does vary. → variable star.
Math.: A symbol, usually a letter, which in a mathematical or logic expression represents any member of a set of possible values or possible objects.

Etymology (EN): M.E., from O.Fr. variable, from L. variabilis “changeable,” from variare “to change,” → vary.

Etymology (PE): Vartandé agent noun from vartidan “to change,” → vary.