1One by one, in order, or separately — describing something done individually rather than all at once.
先生は生徒の名前をいちいち呼んで出席を確認した。
The teacher checked attendance by calling out each student's name one by one.
荷物をいちいち箱に詰めてから発送した。
I packed the items into boxes one by one before shipping them.
彼はメールをいちいち確認してから返信する。
He checks each email individually before replying.
2Every single one, down to the smallest detail — often used with a critical, exasperated nuance about someone's behavior ("every little thing").
そんなにいちいち文句を言わなくてもいいでしょう。
You don't have to complain about every little thing like that.
母は私のすることにいちいち口を出す。
My mother comments on everything I do.
彼は他人の失敗をいちいち覚えている。
He remembers every single mistake other people make.