Something that happens only once in a lifetime; a once-in-a-lifetime opportunity or event. (Note: 「一生に一度」 is the more common spoken form.)
一生一度のチャンスを逃さないように準備しなさい。
Prepare so you don't miss this once-in-a-lifetime opportunity.
あの山に登るのは一生一度の経験だった。
Climbing that mountain was a once-in-a-lifetime experience.
結婚式は多くの人にとって一生一度の儀式だ。
For many people, a wedding is a once-in-a-lifetime ceremony.
言い方としては『一生に一度』が一般的だが、『一生一度』も辞書に載る表現だ。
The phrase '一生に一度' is more common, but '一生一度' also appears in dictionaries.