De*clen"sion (?), n. [Apparently corrupted fr. F. déclinaison, fr. L. declinatio, fr. declinare. See Decline, and cf. Declination.] 1. The act or the state of declining; declination; descent; slope.

The declension of the land from that place to the sea.

2. A falling off towards a worse state; a downward tendency; deterioration; decay; as, the declension of virtue, of science, of a state, etc.

Seduced the pitch and height of all his thoughts
To base declension.

3. Act of courteously refusing; act of declining; a declinature; refusal; as, the declension of a nomination.

4. (Gram.) (a) Inflection of nouns, adjectives, etc., according to the grammatical cases. (b) The form of the inflection of a word declined by cases; as, the first or the second declension of nouns, adjectives, etc. (c) Rehearsing a word as declined.

&fist; The nominative was held to be the primary and original form, and was likened to a perpendicular line; the variations, or oblique cases, were regarded as fallings (hence called casus, cases, or fallings) from the nominative or perpendicular; and an enumerating of the various forms, being a sort of progressive descent from the noun's upright form, was called a declension. Harris.

• (grammar): A way of categorizing of nouns, pronouns, or adjectives according to the inflections they receive.
        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.
  
• (grammar): The act of declining a word; the act of listing the inflections of a word in order.