expression, noun
one ten-thousandth
Used in mathematical, statistical, or figurative contexts to express a very small fraction or probability.
この薬の副作用が出る確率は万分の一です。
The probability of experiencing side effects from this medicine is one in ten thousand.
Compound of 万 (man, 'ten thousand'), 分 (bun, 'part'), and 一 (ichi, 'one'), literally 'one part of ten thousand'.