The Collaborative International Dictionary of English v.0.48:

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.
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            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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WordNet (r) 3.0 (2006):

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"