noun (n)

• the inflection of nouns and pronouns and adjectives in Indo-European languages(noun.linkdef)

  source: wordnet30
• process of changing to an inferior state(noun.process)

  Synonym:
  decline in quality, deterioration, worsening
  

  source: wordnet30
• a downward slope or bend(noun.object)

  Synonym:
  declination, decline, declivity, descent, downslope, fall
  Antonym:
  acclivity, ascent, climb, raise, rise, upgrade
  

  source: wordnet30
• a class of nouns or pronouns or adjectives in Indo-European languages having the same (or very similar) inflectional forms(noun.group)

  Example:
  The first declension in Latin.
  

  source: wordnet30
• The act or the state of declining; declination; descent; slope.(noun)

  source: webster1913