declension

noun

1. (countable, uncountable)A falling off, decay or descent.

   “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.”

   “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.”

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

   “a page full of declensions”

4. (countable, uncountable)A 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.”