Look up downrange in Wiktionary, the free dictionary.
Downrange is the horizontal distance traveled by a spacecraft, or the spacecraft's horizontal distance from the launch site. More often, it is used as an adverb or adjective specifying the direction of that travel being measured in a horizontal direction.
In military slang, downrange is a term for being deployed overseas, usually in a war zone. It is also the name of a comic strip published in the newspaper Stars and Stripes.