Source : Webster's Revised Unabridged Dictionary (1913)

Father-in-law \Fa"ther-in-law`\, n.; pl. {Fathers-in-law}.
   The father of one's husband or wife; -- correlative to
   son-in-law and daughter-in-law.

   Note: A man who marries a woman having children already, is
         sometimes, though erroneously, called their
         father-in-law.

Source : WordNet®

father-in-law
     n : the father of your spouse
     [also: {fathers-in-law} (pl)]