equate v.

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.

1670s, back-formation from equability or else from Latin aequabilis "equal, consistent, uniform, not varying" from aequare "make uniform" (see equate). Related: Equably; equableness.

late 14c., from Medieval Latin aequator (diei et noctis) "equalizer (of day and night)," agent noun from Latin aequare "make equal" (see equate). When the sun is on the celestial equator, twice annually, day and night are of equal length. Sense of "celestial equator" is earliest, extension to "terrestrial line midway between the poles" first recorded in English 1610s.