Why it's called a down

American football did not diverge from rugby gradually. It diverged because of two specific rule changes, proposed by one man, two years apart.

1880: the line of scrimmage

Walter Camp persuaded the student-led rules convention to let a team retain possession after a player was tackled and ruled "down." That single change replaced rugby's contested scrummage — where possession was fought for — with an orderly line of scrimmage, and a team that could plan what to do next.

1882: the problem with that

Guaranteed possession created an obvious exploit: a team that was ahead could simply keep the ball and decline to advance. Camp's answer, accepted on 12 October 1882, was a limit — three downs to gain five yards, or surrender the ball.

Possession stopped being a prize and became a lease.

Everything since is an argument about two numbers

In 1906 the distance doubled to ten yards. In 1912 a fourth down was added. Those two numbers — how many attempts, how far — are the entire tension of the sport, and every strategic argument in football, including the modern fourth-down revolution, is a negotiation over them.

So the word is literal. A "down" is one of your allotted attempts, named after the moment a player was ruled down and the play stopped instead of the ball being fought over.