Definition of declension:
(n) :
a way of categorizing
nouns,
pronouns, or
adjectives according to the inflections they receive; the act of
declining a word; the act of listing the
inflections of a noun, pronoun or adjective in order
"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."