NOUN (2)

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;

The Collaborative International Dictionary of English v.0.48:

Touchdown \Touch"down`\ (t[u^]ch"doun`), n. (Football)
   The act of touching the football down behind the opponents'
   goal; also the score (6 points) resulting from such a play.
   [1913 Webster]

   Safety touchdown. See under Safety.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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