An adverb meaning 'without noticing' or 'before one knew it,' indicating that something happened or changed unnoticed.
いつの間にこんなに暗くなったの？
When did it get so dark (without me noticing)?
彼はいつの間にか会社の中心人物になっていた。
He had somehow become a key person in the company before anyone noticed.
いつの間にか猫が部屋に入ってきていた。
A cat had slipped into the room before I noticed.
いつの間にか締め切りが過ぎてしまった。
The deadline had passed before I knew it.