touchdown

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. 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. n. The contact, or moment of contact, of a landing aircraft or spacecraft with the landing surface.

Century Dictionary and Cyclopedia

1. n. In foot-ball, the touching of the ball to the ground by a player behind his opponent's goal; the play by which this is done.

Wiktionary

1. n. American football, Canadian football A six-point score occurring when the ball enters possession of a team's player in the opponent's end zone. Analogous to a try in rugby.
2. n. aviation The moment when an aircraft makes first or final contact with the ground during a landing.
3. n. rugby A try

GNU Webster's 1913

1. n. (Football) The act of touching the football down behind the opponents' goal; also the score (6 points) resulting from such a play.

WordNet 3.0

1. v. come or bring (a plane) to a landing
2. n. a landing (as the wheels touch the landing field); especially of airplanes
3. n. a score in American football; being in possession of the ball across the opponents' goal line