Meaning

noun a condition inferior to an earlier condition; a gradual falling off from a better state

noun a downward slope or bend

noun (astronomy) the angular distance of a celestial body north or to the south of the celestial equator; expressed in degrees; used with right ascension to specify positions on the celestial sphere

originFrom Middle English declinacioun, borrowed from Middle French declination, from Latin declinatio. Doublet of declension.

Etymology from Wiktionary, CC BY-SA 3.0