時間・時点N3
〜間に（あいだに）
during... (at some point within)
Structure
V-ている + 間に；N + の + 間に
Explanation
Indicates a one-time action or change occurring at some point within a period. Unlike 〜間, 〜間に does not require continuity throughout the whole period.
Examples
留守の間に、誰かが来たようです。
It seems someone came while I was away.
昼寝をしている間に、雨が降り始めました。
It started raining while I was napping.
Note
With 〜間に the main clause is a one-time action or change; with 〜間 it is continuous. Similar to 〜うちに.