Source : Webster's Revised Unabridged Dictionary (1913)

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.

            The declension of the land from that place to the
            sea.                                  --T. Burnet.

   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.                      --Shak.

   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.

   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.

   {Declension of the needle}, declination of the needle.

Source : WordNet®

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}, {worsening}]
     3: a downward slope or bend [syn: {descent}, {declivity}, {fall},
         {decline}, {declination}, {downslope}] [ant: {ascent}]
     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"