noun (n)

• change toward something smaller or lower(noun.process)

  Synonym:
  decline
  

  source: wordnet30
• the statement of a theme in notes of lesser duration (usually half the length of the original)(noun.communication)

  Antonym:
  augmentation
  

  source: wordnet30
• the act of decreasing or reducing something(noun.act)

  Synonym:
  decrease, reduction, step-down
  Antonym:
  increase, step-up
  

  source: wordnet30
• The act of diminishing, or of making or becoming less; state of being diminished; reduction in size, quantity, or degree; -- opposed to augmentation or increase.(noun)

  source: webster1913