equilibrium n.

late 14c., "identical in amount, extent, or portion;" early 15c., "even or smooth of surface," from Latin aequalis "uniform, identical, equal," from aequus "level, even, flat; as tall as, on a level with; friendly, kind, just, fair, equitable, impartial; proportionate; calm, tranquil," which is of unknown origin. Parallel formation egal (from Old French egal) was in use late 14c.-17c. Equal rights is from 1752; by 1854 in American English in reference to men and women. Equal opportunity (adj.) in terms of hiring, etc. is recorded by 1925.

zodiac constellation represented by a pair of scales, late Old English, from Latin libra "a balance, pair of scales," also "pound (unit of weight)," from Proto-Italic *leithra- "pound." De Vaan compares Greek litra "name of a Sicilian coin," which "was probably borrowed from an Italic language at the stage containing [-thr-]."

Not a separate constellation in ancient Greece, where it was khelae, "the claws" of adjacent Skorpios. Nativized in Old Norse as skala-merki. Meaning "person born under the sign of Libra" is from 1894. Related: Libral; Libran.

"absence or destruction of equilibrium," 1840; see dis- + equilibrium. Specific sense in economics is by 1927.