Red Shirt

A "red shirt" or redshirt in videogames, television, and other media is a character who enters the story for the sole purpose of dying early on. By and large, they have little in the way of personality or backstory, and often go unnamed. If they do get a few lines of dialogue, they are usually new in some way (ex. a rookie in the army, a fresh transfer from another company, etc.). The death of redshirt is a plot device used to demonstrate the threat posed to the heroes and/or to motivate the main characters to vengeance for their fallen and little-known comrade without having to kill off anyone important. A technical term for a red shirt is an expendable.

Red shirts are not to be confused with mooks, which are the villain equivalent.

BackgroundEdit
The trope was popularized and named by Star Trek, which killed almost any character wearing (you guessed it) a red shirt to introduce the enemy for that episode. Since everyone else on the show had a shirt color-coded to their station, anyone who was not a central cast member (aka anyone with the title 'Ensign') and wore a red shirt was more or less guaranteed to die.

Another example are the Clone troopers and later Storm Troopers from the Star Wars saga. They are known for dying very early in the story and are often used as cannon fodder.