一旦【いったん】

一旦 is an adverb with two senses. Before a conditional – きごう〜たら, きごう〜と, きごう〜ば – it means "once": after that point is passed, something follows that is hard to undo. Before a plain action it means "for now", a step taken before something else.

It is written in kanji or as いったん, and you hear both senses in conversation as well as read them.

"Once"

The clause with 一旦 sets a threshold, and the second half says what follows from crossing it, usually something lasting or hard to reverse. The verb in the 一旦 clause is a conditional; きごう〜たら, きごう〜と and きごう〜ば all work here, and きごう〜からには or きごう〜いじょう以上 add "and so you must".

"For now"

Without a conditional, 一旦 marks the action as a pause or a first step. Another action is expected afterwards, often in the same sentence with きごう〜てから.

一旦 or 一度

いちど一度 can stand in for 一旦 in the "once" sense: いちど一度きめたら決めたら means the same as 一旦決めたら. 一度 also counts: "one time". 一旦 never counts, so it cannot say how many times something happened.

• × 日本には一旦行ったことがあります → 日本には一度行ったことがあります