the inflection of nouns, pronouns, and adjectives for categories such as case and number.
b.
the whole set of inflected forms of such a word, or the recital thereof in a fixed order.
c.
a class of such words having similar sets of inflected forms: the Latin second declension.
2.
an act or instance of declining.
3.
a bending, sloping, or moving downward: land with a gentle declension toward the sea.
4.
deterioration; decline.
5.
deviation, as from a standard.
Origin: 1400–50; late Middle English declenson, declynson (with suffix later assimilated to -sion), by stress retraction and syncope < Old French declinaison < Latin dēclīnātiōdeclination
1560s, a grammatical term, ult. from L. declinationem, noun of action from declinare (see decline); perhaps via French; "the form is irregular, and its history obscure" [OED].