The Collaborative International Dictionary of English v.0.48:

Betrothal \Be*troth"al\, n.
   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. "The
   feast of betrothal." --Longfellow.
   [1913 Webster]

WordNet (r) 3.0 (2006):

betrothal
    n 1: a mutual promise to marry [syn: betrothal, troth,
         engagement]
    2: the act of becoming betrothed or engaged [syn: betrothal,
       espousal]