A "redshirt" is a fan-derived term which refers to a specific class of character featured in the Star Trek mythos. Redshirts are Federation crew members who are considered expendable and usually only make one appearance. Often times they are selected for a specific away mission in which they are killed. Though these "cannon fodder" characters stem from nearly any department on a starship, more often than not, they are either specialists or tactical officers who wear red tunics.
Returnee is an informal term referring to people who were victims of abduction and later returned to their place (or time) of origin. The term is primarily applied to the 4,400 human beings who were abducted at different points in time and returned to the base of Mount Rainier in the 2004-07 television series The 4400.