Once; on one occasion — used to indicate that something happens one time or to mark the first of a sequence.
いったん家に帰ってから、また出直します。
I'll go home once and then come back again.
問題をいったん整理してから、話し合いましょう。
Let's organize the issues once first, then discuss them.
彼はその場をいったん離れた。
He left the place for a while (left once).
Temporarily; for a short time; briefly — used to indicate a temporary or brief state.
一旦パソコンの電源を切って、再起動してください。
Please turn off the computer once and then reboot it.
混雑が収まるまで、いったん待合室でお待ちください。
Please wait in the waiting room for a short time until the crowd eases.
その件は一旦保留にしましょう。
Let's put that matter on hold for the time being.
An archaic noun meaning 'one morning' (rarely used in modern Japanese).
一旦、里へ参りし人ありけり（古風な用例）.
Once, there was someone who went to the village. (archaic example)