Definition of "declension"

Refinement of feeling, intellectual tastes, and a noble hospitality, were among the features of his character; and hoary years brought no mental declension, and drew no shade over the ardent affections by which he was distinguished, and in whose reciprocity, was his undeclining solace.

1845, Lydia Sigourney, Scenes in my Native Land, The Great Oak of Geneseo, page 86

The custom of rolling a burning wheel down a hill […] might well pass for an imitation of the sun's course in the sky, and the imitation would be especially appropriate on Midsummer Day when the sun's annual declension begins.

1890, James George Frazer, The Golden Bough, volume 2, page 268