Is it ensure, insure, or assure?
1893, noun of action from prorate (v.).
also pro-rate, "divide proportionally," 1860, American English, verb derived from Latin pro rata (parte) (see pro rata). Related: Prorated; prorating.
To divide or distribute a sum of money proportionately. For example, if one owned an automobile for only three months, an insurance company would prorate the annual premium by charging only one-quarter of it.