Redshirt

Service (Security, Engineering) crew in the origional Star Trek. They were Expendable Crew Members and died rapidly on Away Missions. They have swapped colours with command crew and are now Yellowshirts.

In college athletics in the US:
When a college athlete takes a year off from playing his or her sport. This is done to preserve a year of that student's NCAA eligibility (which is normally four years.) Usually an athlete is redshirted because they are not likely to play that year due to injury (i.e. they're hurt), grades (i.e. they're dumb) or that there are already better players on the team (i.e. they suck.)

Besides the hapless Star Trek versions, there are red shirts in the US Navy.

The red shirt, accompanied by a red semi-helmet and a red life vest, are the folks who are there in case of a catastrophic emergency. Their job is to put out any fires and to rescue any endangered people, such as pilots or green shirts. Usually there are at least two in full flame-proof silver suits (with the required red shirt underneath, of course).

These sailors are rarely needed, and their job is to sit and wait just in case. They get full flight deck pay allowances, since their job involves hazardous duty. There have been incidents (such as the USS Forrestal) where a crashed aircraft set off multiple bombs and ammunition, which was all captured on tape and is played at every firefighting class. One minute there are folks fighting a fire, then a flash, then just bits of hoses flailing about randomly.

Just like their Star Trek counterparts, they just don't know when something bad will happen.

"No, he's not playing this year. He's a red shirt."

A college athlete in the NCAA is given 5 years from the time of enrollment to participate in 4 competitive seasons of any and all of the athletic programs available at the school. So as soon as you start school, you're burning your time. Some people choose not to compete during their first year, in order to gain experience before beginning their 4 years of real competition. These individuals are commonly referred to as "red shirt freshmen".

The red shirt rule was ostensibly written to provide for a year of recovery time, should an athlete sustain injuries which preclude competition in his/her sport.

The term Redshirt, taken from the Star Trek redshirts, now refers to the general trope of expendable minor characters. Killing off minor characters is a good (albeit overused) way of raising tension, underlining the seriousness of the situation, and generally advancing the plot. Redshirts are common in adventure, science fiction/fantasy, and dark comedy... and often overwhelming in their sheer mass when these genres combine. The phrase redshirt army may be used when the body count rises to cannon fodder levels.

Redshirts are often defined as specifically the good guys -- bad guys have mooks instead -- but pretty much anyone can be a redshirt, as long as their death is a Bad Thing, rather than a Good Thing.

The phrase mauve shirt is sometimes used to describe a minor character whose status is uncertain, usually because while they are a comparatively minor character, they have been around for long enough that we are starting to get attached. They may simply be a reserve redshirt saved for a dramatic death later on, or they may indeed survive, depending on the author's needs.