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.