Source : Webster's Revised Unabridged Dictionary (1913)

Touchdown \Touch"down`\, n. (Football)
   The act of touching the football down behind the opponents'
   goal .

   {Safety touchdown}. See under {Safety}.

Source : WordNet®

touchdown
     n 1: a score in American football; being in possession of the
          ball across the opponents' goal line
     2: a landing (as the wheels touch the landing field);
        especially of airplanes