noun (n)

• a mutual promise to marry(noun.communication)

  Synonym:
  engagement, troth
  

  source: wordnet30
• the act of becoming betrothed or engaged(noun.act)

  Synonym:
  espousal
  

  source: wordnet30
• The act of betrothing, or the fact of being betrothed; a mutual promise, engagement, or contract for a future marriage between the persons betrothed; betrothment; affiance.(noun)

  source: webster1913