noun (n)

• two items of the same kind(noun.quantity)

  Synonym:
  brace, couple, couplet, distich, duad, duo, dyad, pair, span, twain, twosome, yoke
  

  source: wordnet30
• two performers or singers who perform together(noun.group)

  Synonym:
  duette, duo
  

  source: wordnet30
• a pair who associate with one another(noun.group)

  Synonym:
  couple, duo, twosome
  

  source: wordnet30
• a musical composition for two performers(noun.communication)

  source: wordnet30
• (ballet) a dance for two people (usually a ballerina and a danseur noble)(noun.act)

  Synonym:
  pas de deux
  

  source: wordnet30
• A composition for two performers, whether vocal or instrumental.(noun)

  source: webster1913