What is a base period employer?

Unemployment compensation is usually paid on the basis of wages earned in the "base period", which is generally the first 4 quarters of the last 5 quarters of wages completed. A "base period employer" is one you worked for in that period, who's account would be charged by the unemployment office through unemployment taxes. There could be more than one employer with that designation, depending on how many you worked for. Employers pay smaller taxes if their turnover rate is low as an incentive to retain employees.