一旦・いったん

• Reading: いったん
• Romaji: ittan
• Formation: いったん + Verb conditional (と/たら/ば) + Consequence / いったん + Action (temporary)
• Kanji breakdown: 一 = one; 旦 = dawn, morning

Meaning

An adverb meaning 'once' or 'once something happens,' indicating that after a condition is established, it is difficult to reverse or has significant consequences. It can also mean 'for the time being' or 'temporarily.'

いったん has two primary uses. In the first, it means 'once (something happens)' and is followed by a conditional (と, たら, ば) to express that after a threshold is crossed, the consequences are serious or irreversible. In the second, it means 'for now' or 'temporarily,' indicating a provisional pause before resuming. The first usage is more common in formal and written Japanese and often appears in warnings, rules, and proverbs. The second usage is common in everyday speech, especially in workplace settings. Unlike ひとまず, which is more casual and implies 'just for starters,' いったん suggests a deliberate, calculated pause.

Examples

1. いったん信頼を失うと、取り戻すのは容易ではない。 Once you lose trust, it is not easy to regain it.
2. 火災がいったん広がれば、消火は極めて困難になる。 Once a fire spreads, extinguishing it becomes extremely difficult.
3. いったんここで作業を中断して、午後から再開しましょう。 Let's pause the work here for now and resume in the afternoon.

Usage Guide

Context: spoken, written, everyday, business

Tone: cautionary

Do Say

• いったん約束した以上、守らなければならない。
• 交渉はいったん中断し、来週改めて話し合うことにした。
• いったん悪い癖がつくと、なかなか直せない。
• いったん帰宅してから、改めて出かけます。

Don't Say

• いったん毎日走っている。(Using いったん for ongoing habitual actions — it marks a single threshold event or a temporary pause) → 毎日走っている。
• いったん昨日会議があった。(Using いったん to describe a simple past event — it requires a conditional consequence or a temporary action) → 昨日、会議がいったん中断された。

Origin & History

一旦 is a Sino-Japanese compound from 一 (one) and 旦 (dawn, morning). It originally meant 'one morning' and by extension 'a brief moment,' which developed into the meanings of 'once (a threshold is crossed)' and 'temporarily.'

Cultural Context

Generation: All ages

Social background: Universal