noun

1. A Member of Parliament who does not have cabinet rank, and who therefore sits on one of the backbenches or in one of the back rows of the legislature.

2. A student who does not perform well, especially one who sits at the back of the classroom.

3. A member of a team who does not usually play, but who is held in reserve.

4. (by extension) Someone who does not play an active role in a process.