one by one

Từ điển WordNet v3.1 - WordNet Dictionary

adverb

1. in single file (Freq. 6)
- the prisoners came out one by one
• Syn:
one after another, one at a time
2. one piece at a time (Freq. 1)
- she sold the plates by the piece
• Syn:
by the piece
3. apart from others
- taken individually, the rooms were, in fact, square
- the fine points are treated singly
• Syn:
individually, separately, singly, severally, on an individual basis
• Derived from adjective:
single (for: singly), separate (for: separately), individual (for: individually)