Etymology
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equator (n.)

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.

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Definitions of equator

equator (n.)
an imaginary line around the Earth forming the great circle that is equidistant from the north and south poles;
the equator is the boundary between the northern and southern hemispheres
equator (n.)
a circle dividing a sphere or other surface into two usually equal and symmetrical parts;
From wordnet.princeton.edu