From The Collaborative International Dictionary of English v.0.48 [gcide]:
Declension \De*clen"sion\, n. [Apparently corrupted fr. F.
d['e]clinaison, fr. L. declinatio, fr. declinare. See
Decline, and cf. Declination.]
1. The act or the state of declining; declination; descent;
slope.
[1913 Webster]
The declension of the land from that place to the
sea. --T. Burnet.
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2. A falling off towards a worse state; a downward tendency;
deterioration; decay; as, the declension of virtue, of
science, of a state, etc.
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Seduced the pitch and height of all his thoughts
To base declension. --Shak.
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3. Act of courteously refusing; act of declining; a
declinature; refusal; as, the declension of a nomination.
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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.
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Note: 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.
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Declension of the needle, declination of the needle.
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From WordNet (r) 3.0 (2006) [wn]:
declension
n 1: the inflection of nouns and pronouns and adjectives in
Indo-European languages
2: process of changing to an inferior state [syn:
deterioration, decline in quality, declension,
worsening]
3: a downward slope or bend [syn: descent, declivity,
fall, decline, declination, declension, downslope]
[ant: acclivity, ascent, climb, raise, rise,
upgrade]
4: a class of nouns or pronouns or adjectives in Indo-European
languages having the same (or very similar) inflectional
forms; "the first declension in Latin"