adverbial N2 uncommon politewritten

一旦 — once / temporarily

一旦・いったん

Meaning

① ①: once ~ (has happened) — emphasizing that after this single point a consequence inevitably follows

② ②: for the time being / temporarily — pausing or doing something provisionally before resuming

① ①

一旦いったん約束やくそくしたら、必かならず守まもるべきだ。

Once you've made a promise, you must keep it.

② ②

会議かいぎを一旦いったん中断ちゅうだんしましょう。

Let's break off the meeting for now.

Attaches to	Form	Example
Adverb	一旦いったん + clause (sense ① often + たら / と / ば + a consequence)	一旦いったん決きめたら

① ①

一旦いったん火ひがつくと、消けすのは難むずかしい。

Once it catches fire, it's hard to put out.

一旦いったん引ひき受うけた以上いじょう、最後さいごまでやる。

Now that I've taken it on, I'll see it through to the end.

② ②

一旦いったん家いえに帰かえって、出直でなおします。

I'll go home for now and come back later.

雨あめが一旦いったんやんだ。

The rain let up for the moment.

Distinct from 一度いちど 'once, one time,' which counts an occurrence — 一旦いったん marks a turning point or a temporary pause, not a count.
Formal/written-leaning; in casual speech とりあえず or ひとまず often covers sense ②.

Jengo shows 一旦 the way you actually meet it: inside real Japanese sentences, so it sticks instead of staying an abstract rule.

Sources Compiled with reference to A Dictionary of Advanced Japanese Grammar.