The disposable and doomed character from Star Trek, so named because the character is some unnamed ensign. This term is usually applied to yellow (or tan or whatever color that is) shirt crew members from Star Trek: The Next Generation. In Star Trek: The Original Series, this type of character was most often called a red shirt, although the term Ensign Nobody can apply to this as well.
The seventh season Next Generation episode "Lower Decks" was built around the idea that these people do have names and lives.