Meaning

noun the great circle on the celestial sphere midway between the celestial poles

adjective relating to the vicinity of the equator

adjective relating to an equinox (when the lengths of night and day are equal)

originInherited from Middle English equinoctial, equinoccial, equinoxial, from Old French equinoxial, from Latin (circulus) aequinoctiālis, from aequinoctium + -alis.

Etymology from Wiktionary, CC BY-SA 3.0