expression, noun which may take the genitive case particle 'no'
one in a million; extremely unlikely
Used to describe a very slim chance or a highly improbable event. Often appears in conditional expressions like 万に一つも～ない or 万に一つの確率.
宝くじが当たる確率は万に一つもない。
The chance of winning the lottery is not even one in a million.
万に一つの可能性にかけてみる。
I'll bet on that one-in-a-million possibility.
万一 is a more common and compact expression for 'if by any chance' or 'in the unlikely event'. 万に一つ emphasizes the extreme rarity of the event itself.
Literally 'one in ten thousand', from 万 (ten thousand) and 一つ (one). The number 万 is used hyperbolically to express an extremely small probability.