touchdown vs touch down

touchdown is a noun but touch down is not a noun.

Word	Noun	Adjective	Verb	Adverb
touchdown	Yes	No	No	No
touch down	No	No	Yes	No

touchdown (noun)	touch down (noun)
a score in American football; being in possession of the ball across the opponents' goal line
a landing (as the wheels touch the landing field); especially of airplanes

touchdown (verb)	touch down (verb)
	come or bring (a plane) to a landing