noun (n)

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

  Synonym:
  replacement, substitution, switch, transposition
  

  source: wordnet30
• the act of changing the arrangement of a given number of elements(noun.act)

  source: wordnet30
• complete change in character or condition(noun.act)

  Example:
  The permutations...taking place in the physical world.
  

  source: wordnet30
• act of changing the lineal order of objects in a group(noun.act)

  source: wordnet30
• The act of permuting; exchange of the thing for another; mutual transference; interchange.(noun)

  source: webster1913