noun (n)

• the act of pairing a male and female for reproductive purposes(noun.act)

  Synonym:
  conjugation, coupling, mating, sexual union, union
  

  source: wordnet30
• the act of grouping things or people in pairs(noun.act)

  source: wordnet30
• The act or process of uniting or arranging in pairs or couples.(noun)

  source: webster1913