"Pro rata" is a Latin term meaning "in proportion." It is commonly used in finance, accounting, and various fields to describe the allocation or distribution of something proportionally based on a specific factor, often relating to time, amount, or another measurable standard. For example, if you divide an amount of money or a set of responsibilities among several parties in proportion to their contributions or stakes, you are doing it on a pro rata basis.
"Pro rata" comes from Latin, where "pro" means "for" and "rata" is the feminine form of "ratus," meaning "reckoned" or "calculated." Thus, it translates to "for that which is reckoned."