declension

plural declensions

1. countableuncountableA falling off, decay or descent.

   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

   1 more example

   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

2. countableuncountablegrammarThe act of declining a word; the act of listing the inflections of a noun, pronoun or adjective in order.
3. countableuncountablegrammarThe product of that act; a list of declined forms.

   a page full of declensions

4. countableuncountablegrammarA way of categorizing nouns, pronouns, or adjectives according to the inflections they receive.

   In Latin, 'amicus' belongs to the second declension. Most second-declension nouns end in '-i' in the genitive singular and '-um' in the accusative singular.