WordWeb Online

1. In succession
   "the prisoners came out one by one";
   - one at a time
    
2. One piece at a time
   "she sold the plates one by one";
   - by the piece
    
3. Apart from others
   "taken one by one, the rooms were, in fact, square";
   - individually, separately, singly, severally, on an individual basis

Encyclopedia: One by one