An appositive is a noun that renames the subject of a sentence. Appositives are usually set off by commas, but other separators such as dashes can be used. An appositive doesn't have to be a single noun, it can have its own modifiers. The use is normally when one needs to justify or explain the reason for the action taking place in the sentence. Note that only the appositive is set off by commas. If the subject comes after the appositive, it does not get a comma or other separator after it.
In a Reed-Kellogg Diagram, appositives are enclosed in parenthesis.
- ZIM, a little green alien, made plans for the destruction of Earth. # "ZIM" is the subject, "a little green alien" is the appositive
- A firefighter, Bob works odd shifts. # "Bob" is the subject, "A firefighter" is the appositive
- Samara -- a green pimply ogre -- ate a few babies. # "Samara" is the subject, "a green pimply ogre" is the appositive
See also: Appositive phrase