What is Declension?

Declension definition and meaning on Dictionary terms:
Grammar. the inflection of nouns, pronouns, and adjectives for categories such as case and number. the whole set of inflected forms of such a word, or the recital thereof in a fixed order. a class of such words having similar sets of inflected forms: the Latin second declension.

an act or instance of declining.
a bending, sloping, or moving downward: land with a gentle declension toward the sea.
deterioration; decline.
deviation, as from a standard.