In the Writer’s Glossary today…
This term originates from Star Trek, but is often found in printed fiction, too. A redshirt is a character destined to die soon after making their entrance. In the TV series, characters in red uniforms (usually security or engineering) would often meet some grisly fate before the end of the second act. This happened so frequently that the term Redshirt became a trope for any character who was not going to survive.
Such characters are not essential to the plot – thus aren’t usually provided with names, histories or personalities. They are, however, useful devices for proving that a situation is dire. They can be used in foreshadowing or so-called ‘small scenes’, to show the reader the consequences or dangers associated with a situation or character. They are cannon-fodder for scenes where someone has to die to make the plot believable, while allowing the main characters to survive.