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e·qua·tor (i-kwātər)
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n.
1.
a. The imaginary great circle around the earth's surface, equidistant from the poles and perpendicular to the earth's axis of rotation. It divides the earth into the Northern Hemisphere and the Southern Hemisphere.
b. A similar great circle drawn on the surface of a celestial body at right angles to the axis of rotation.
2. The celestial equator.
3. A circle that divides a sphere or other surface into congruent parts.

[Middle English, fromMedieval Latinaequātor (diei et noctis), equalizer (of day and night), fromLatinaequāre, to equalize; see EQUATE.]
(click for a larger image)
equator

The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2022 by HarperCollins Publishers. All rights reserved.
 

Indo-European & Semitic Roots Appendices

    Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:

    Indo-European Roots

    Semitic Roots

    The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.