The Collaborative International Dictionary of English v.0.48:

Cotillon \Co`til`lon"\ (k[-o]`t[-e]`y[^o]N" or k[-o]`t[-e]l`-;
   277), Cotillion \Co*til"lion\ (k[-o]*t[i^]l"y[u^]n), n. [F.
   cotillon, fr. OF. cote coat, LL. cotta tunic. See Coat.]
   1. A brisk dance, performed by eight persons; a quadrille.
      [1913 Webster]

   2. A tune which regulates the dance.
      [1913 Webster]

   3. A kind of woolen material for women's skirts.

   4. A formal ball, especially one at which debutantes are
      first presented to society.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

cotillion
    n 1: a ball at which young ladies are presented to society [syn:
         cotillion, cotilion]
    2: a lively dance originating in France in the 18th century
       [syn: cotillion, cotilion]