as in ghost producer, ghost songwriter, ghost writer (books) etc.
that means when the person remains anonymous or doesn't take credit for his/her work.
1) Kanye "ghost-produced" for rapper D-Dot before.
2) Ne-Yo ghost-wrote for Beyonce on her song Irreplaceable and everybody thought Beyonce had written the song.
3) Much of the later part of the children’s horror series Goosebumps was ghostwritten.