Another great definition from the mighty Urban Dictionary:

An employee who maintains a position at a company despite the fact that all of his job duties have been reassigned to other employees. This could be by the ghost’s own design or due to restructuring within the department.

Office Ghosts are particularly effective when they have a big title that comes with an assistant, and a boss in another location because no one is really sure what they do all day, but they still have the ability to step up and take credit once a project is completed.

Employee: I was looking for Michael. Have you seen him?
Assistant: I think he’s in his office but I’m not sure since he always has the door closed. Let me check.
Assistant: No, he’s not in there. He might be in a meeting.
Employee: He’s like the office ghost. I can never find him when I need to talk with him. What does he do all day?
Assistant: Honestly I have no idea. You tell me when you figure it out and we’ll both know.