The basic definition of the adverbial noun 一旦 is "once." This meaning derives from referring to a brief period of time. In fact, the Kanji 旦 originally means "morning," which is why it is also seen in the word 元旦 for "New Year."
Grammatically speaking, 一旦 can be viewed as a basic vocabulary word with little strings attached to understanding it properly as the ways in which means "once" mirror its English equivalent. Nonetheless, we will be taking the time to examine it closely and compare it to similar phrases.
The basic meaning of the noun 一旦 is "once," being synonymous with 一度.
Cancellation had been decided at one time.
Literally: Not asking is a single embarrassment, but not asking is an embarrassment that lasts for eternity.
Better to ask and be embarrassed than not ask and never know.
3a. I want to stay (there) at least once.
3b. I want to stay (there) at least [this once/for now].
一旦 may also mean "temporary," in which case it is synonymous with 一時的. When seen as 一旦は, its true colors as an adverbial noun appear, becoming synonymous with しばらくは.
A temporary pause is unavoidable.
With that, things were at a standstill for a moment, but afterwards, the last one-to-one fight began.
I was given a violation ticket for temporarily stopping today, but it's just too hard for me to swallow.
7. 一旦の楽しみに溺れ、長き一生を過つは、極めて口惜しきことに候はずや。(Classical Japanese)
Drowning in temporary fun and messing up the rest of your long life, how incredibly vexing that must be!
Meaning ①: "Once"
The pattern いったん X ［と・たら・ば] translates to "once" X starts, then...", implying that once X happens, it becomes hard to revert back to the previous state.
Once that dragon starts lashing out, you can no longer put your hands on it.
Once Taylor starts playing a game, he can't seem to stop.
Once you've begun, you mustn't quit.
Once you start using a smart phone, you can't go back to using a flip phone, you know?
Since that's the sort of skill that you can use for the rest of your life once you're capable of it, if you keep on improving on it, I think there's a chance that you'll make big on it.
In fact, 一旦 also shares this meaning of "once" with 一度, which can be read as either いちど or ひとたび, with the latter being more literary in tone.
This is the sort of flavor you can't forgive once you've tried it.
Once you start living in Tokyo, you'll definitely get used to it.
Once you start smoking, you'll get addicted.
Once you've set your mind to it, of course, see it through.
Meaning ②: "For Now"
Another meaning that the adverb 一旦 has is to mean "for now/temporarily," in which case it is synonymous with 一先ず and 一時的に. The nuance that 一旦 has is that it marks a transition point between the current situation about to that point and the next which is likely to be temporary but needs to take place nonetheless. This is often used when leading others into the next course of action even if it isn't the final solution.
Comparison Note: 一先ず brings up the next course of action although it isn't quite what one might want. It's "for now" as in "this is what we're doing for the time being." 一時的に is quite literally "temporarily" and doesn't indicate inherently clue in the listener as to how the speaker views the situation.
Pronunciation Note: When 一旦 is used as a noun, it has a HLLL, but when it is used as an adverb "for now," its pitch accent is LHHH.
For now, let's split up.
I'll be going home for now.
If (they) were to explain their future plans to the citizens for now, they will recover trust (in them).
I'll take it back (with us) for now and perform an inspection.