Source : Webster's Revised Unabridged Dictionary (1913)

Cedilla \Ce*dil"la\, 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.

Source : WordNet®

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