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late 14c. as a term in astronomy, from Old French declinacion (Modern French déclinaison), from Latin declinationem (nominative declinatio), noun of action from past participle stem of declinare (see decline). It took on various other senses 15c.-17c., most now obsolete.
declination dec·li·na·tion (děk'lə-nā'shən)
A bending, sloping, or other deviation from a normal vertical position.
A deviation of the vertical meridian of the eye to one or the other side due to rotation of the eyeball about its anteroposterior axis.