one by one in WordNet English dictionary

1. one piece at a time
   "she sold the plates by the piece"
   by the piece  one by one  by the piece  one by one 
   
2. apart from others
   "taken individually, the rooms were, in fact, square"; "the fine points are treated singly"
   individually  separately  singly  severally  one by one  on an individual basis  individually  separately  singly  severally  one by one  on an individual basis 
   
3. in single file
   "the prisoners came out one by one"
   one by one  one after another  one at a time  one by one  one after another  one at a time 
   

one by one in English Wiktionary