also: ひとたび
I have met him only once.
Once I start, I see it through to the end.
temporarily; for a time; for a moment
Only with reading ひとたび
Restricted to the reading ひとたび. This is an uncommon, somewhat literary adverbial use meaning that a state holds only briefly or for a limited time.
The fighting subsided for a time, but spread again the following year.
one degree; one tone; one musical interval
Only with reading いちど
Restricted to the reading いちど. Used for one degree in measurements such as temperature or angles; in music, it can refer to a first-degree or unison-type interval.
The temperature fell by one degree compared with yesterday.
Two notes of the same pitch are called a unison interval.
I'd like to try going to that shop once.
Today the temperature rose by one degree.
He memorized the kanji in just one try.
Once a war breaks out, many people suffer.
一度 is the standard, neutral word for 'once'; いっぺん is a more casual, colloquial variant used mainly in speech.
一度 just means 'one time' with no implication of scale, whereas 一挙 implies a sweeping action that resolves or changes many things at once.
一度 and 三度 differ only in the number kanji (一 vs 三), so learners can easily misread 'once' for 'three times'.
first-degree burn
Practical kana spelling for the more formal or literary reading ひとたび.
Mixed kanji-kana spelling for ひとたび; seen in formal or literary contexts.
Arabic-numeral spelling; especially common for measurements such as temperature or angles, and also understandable for 'one time'.
Rare mixed spelling for ひとたび; useful mainly as a recognition variant.
Form marked as irregular okurigana; not recommended as a normal modern spelling.