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.
      [1913 Webster]

            The declension of the land from that place to the
            sea.                                  --T. Burnet.
      [1913 Webster]

   2. A falling off towards a worse state; a downward tendency;
      deterioration; decay; as, the declension of virtue, of
      science, of a state, etc.
      [1913 Webster]

            Seduced the pitch and height of all his thoughts
            To base declension.                   --Shak.
      [1913 Webster]

   3. Act of courteously refusing; act of declining; a
      declinature; refusal; as, the declension of a nomination.
      [1913 Webster]

   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.
          [1913 Webster]

   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.
         [1913 Webster]

   Declension of the needle, declination of the needle.
      [1913 Webster]

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"

Moby Thesaurus II by Grady Ward, 1.0:

149 Moby Thesaurus words for "declension":
   IC analysis, abnegation, accidence, affix, affixation, allomorph,
   bound morpheme, cascade, catabasis, cataract, chute, collapse,
   comedown, conjugation, contradiction, crash, cutting, debacle,
   debasement, decadence, decadency, deceleration, declination,
   declinature, decline, decline and fall, declining, decrescendo,
   defluxion, deformation, degeneracy, degenerateness, degeneration,
   degradation, demotion, denial, depravation, depravedness,
   depreciation, deprivation, derivation, derogation, descending,
   descension, descent, deterioration, devolution, difference of form,
   dilapidation, diminuendo, disagreement, disallowance, disclaimer,
   disclamation, disobedience, dissent, dive, down, downbend,
   downcome, downcurve, downfall, downflow, downgrade, downpour,
   downrush, downtrend, downturn, downward mobility, downward trend,
   drop, dropping, dwindling, dying, ebb, effeteness, enclitic,
   fading, failing, fall, falling, falling-off, formative, free form,
   gravitation, holding back, immediate constituent analysis,
   inclination, infix, infixation, inflection, involution, lapse,
   loss of tone, morph, morpheme, morphemic analysis, morphemics,
   morphology, morphophonemics, nay, negation, negative,
   negative answer, nix, no, nonacceptance, noncompliance, nonconsent,
   nonobservance, paradigm, plummeting, plunge, pounce, prefix,
   prefixation, proclitic, radical, rapids, recantation, refusal,
   regression, rejection, remission, repudiation, retention, retreat,
   retrocession, retrogradation, retrogression, root, ruination,
   slippage, slowdown, slump, stem, stoop, subsidence, suffix,
   suffixation, swoop, theme, thumbs-down, turndown, unwillingness,
   wane, waterfall, withholding, word-formation