Why is a touchdown called a touchdown?

Answer 1

In the earliest days of American football, a "touchdown" required that the ball physically touch the ground beyond the goal line. This requirement was removed from the rules in 1889, but the name stuck.

Answer 2

The name comes from Rugby. In that sport, when a player advances the ball into his opponent's end zone, he has to physically touch the ball down to the ground for the score to count. Football originally used the same rule but abandoned it in the early 20th century while retaining the terminology.

Answer 3

Its called a TRY.