pl. n. space marines; also colonial marine
n., in science fiction milieux, a future soldier trained for off-world combat operations, usu. patterned after the modern United States Marine Corps; esp. well-known in gaming milieux associated with the Alien, StarCraft and Warhammer: 40,000 franchises.
Addendum: the term originates in science fiction literature. Credit for inventing the term is often given to Robert A. Heinlein, who used it in 1948’s Space Cadet (I could not locate an earlier reference); the phrase does not appear in 1959’s Starship Troopers, but later space marines, particularly Games Workshop’s Warhammer:40K space marines, are heavily modeled on the precedent set in the latter novel. Brave New Words: The Oxford Dictionary of Science Fiction lists five references to “space marines” in books, stories, and newspapers; only one of these is a Games Workshop reference.
Space marines became a common pop culture trope when they were featured in the 1986 film Aliens; in the film they were referred to as “colonial marines.”
Games Workshop has gotten into trouble with fans (as of early 2013) for attempting to enforce a spurious copyright claim on the term space marines©. This is not the first incident involving GW, space marines, and censorship.