expression, noun which may take the genitive case particle 'no'
Used for rare, special opportunities or experiences that may only happen once in one's life. Often modifies nouns with の.
これは一生に一度のチャンスだ。
This is a once-in-a-lifetime chance.
一生に一度の経験をした。
I had a once-in-a-lifetime experience.
Compound of 一生 (isshō, 'one's whole life') + に (ni, particle) + 一度 (ichido, 'once'). The literal meaning is 'once in a lifetime'.