二十分之一 is a fraction meaning 'one twentieth' or 'one out of twenty'. It is typically used as a noun or adverbial to express a proportion, rate, or part of a whole. Despite its POS label ('m' for measure word), it does not function as a classifier in Chinese — it behaves as a numeric expression of quantity and cannot be used like a true measure word (e.g., 个, 张). Use it in contexts involving statistics, probabilities, or fractions of a group.