The Collaborative International Dictionary of English v.0.48:

cedilla \ce*dil"la\ (s[-e]*d[i^]l"l[.a]), n. [Sp. cedilla, cf.
   F. c['e]dille; dim. of zeta, the Gr. name of the letter z,
   because this letter was formerly written after the c, to give
   it the sound of s.]
   A mark placed under the letter c [thus, [,c]], to show that
   it is to be sounded like s, as in fa[,c]ade.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cedilla
    n 1: a diacritical mark (,) placed below the letter c to
         indicate that it is pronounced as an s