DECLENSION

A falling off, decay or descent.

(grammar) The act of declining a word; the act of listing the inflections of a noun, pronoun or adjective in order.

(grammar) 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."