Definition of declension:
a way of categorizing noun
s, or adjective
s according to the inflections they receive; the act of declining
a word; the act of listing the inflection
s 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."