A touchdown is a means of scoring in both American and Canadian football. Whether running, passing, returning a kickoff or punt, or recovering a turnover, a team scores a touchdown by advancing the ball into the opponent's end zone.
To score a touchdown, one team must take the football into the opposite end zone. The touchdown is scored the instant the ball crosses the plane of the goal line (that is, if any part of the ball is in the space on, above, or across the goal line) while in possession of a player whose team is trying to score in that end zone. The play is dead and the touchdown scores the moment the ball crosses the goal line in possession of a player, or the moment the ball comes into possession of an offensive player in the end zone (having established possession by controlling the ball and having one or both feet or another part of the body on the ground). The slightest part of the ball being over the goal line is sufficient for a touchdown to score. However, only the ball counts, not a player's helmet, foot, or other part of the body. Touching one of the pylons at either end of the goal line with the ball constitutes "breaking the plane" as well.
Touchdowns are usually scored by the offense by running or passing the ball. However, the defense can also score a touchdown if they have recovered a fumble or made an interception and return it to the opposing end zone. Special teams can score a touchdown on a kickoff or punt return, or on a return after a missed or blocked field goal attempt or blocked punt. In short, any play in which a player legally carries the ball across the goal line scores a touchdown, and the manner in which he gained possession is inconsequential. In the NFL, a touchdown may be awarded by the referee as a penalty for a "palpably unfair act" such as a player coming off the bench during a play and tackling the runner who would otherwise have scored.
American football scoring
Conversion (gridiron football)