Emily Dickinson Lexicon

DE-CLEN'SION, n. [L. declinatio, from declino. See Decline.]

1. Literally, a leaning back or down: hence, a falling or declining toward a worse state; a tendency toward a less degree of excellence or perfection. The declension of a state is manifested by corruption of morals. We speak of the declension of virtue, of manners, of taste, of the sciences, of the fine arts, and sometimes of life or years; but in the latter application, decline is more generally used.
2. Declination; a declining; descent; slope; as, the declension of the shore toward the sea. – Burnet.
3. In grammar, inflection of nouns, adjectives and pronouns; the declining, deviation or leaning of the termination of a word from the termination of the nominative case; change of termination to form the oblique cases. Thus from rex in the nominative case, are formed regis in the genitive, regi in the dative, regem in the accusative, and rege in the ablative.