touch·down 
(tŭchdoun′)
(tŭchdoun′)n.
1. Abbr. TD Football An act of carrying, receiving, or gaining possession of the ball across the opponent's goal line for a score of six points.
2. The contact, or moment of contact, of a landing aircraft or spacecraft with the landing surface.
[From the earlier practice of touching the ball to the ground behind the goal line.]