one by one; separately; individually
Describes doing something for each item or person separately, often with a nuance of thoroughness or, in negative contexts, annoyance at having to deal with every single thing.
彼は書類をいちいち確認した。
He checked the documents one by one.
Please don't complain about every little thing.
every single; each and every; without omission
Emphasizes that no item is left out; often used in negative sentences or when expressing that something is done exhaustively, sometimes with a critical tone.
He butts in on every single thing.
There's no need to explain each and every detail.
He complains about every little thing, so it's hard to work with him.
I checked the number of parts one by one before putting them in the box.
My mother nags about every single thing, even how I clean.
I have to explain everything one by one to the new employee.