noun (n)

• an event in which one thing is substituted for another(noun.event)

  Synonym:
  permutation, replacement, switch, transposition
  

  source: wordnet30
• the act of putting one thing or person in the place of another:(noun.act)

  Example:
  He sent Smith in for Jones but the substitution came too late to help.
  

  Synonym:
  commutation, exchange
  

  source: wordnet30
• The act of substituting or putting one person or thing in the place of another; as, the substitution of an agent, attorney, or representative to act for one in his absense; the substitution of bank notes for gold and silver as a circulating medium.(noun)

  source: webster1913