Service to an employer that is recognized for the defined benefit pension plan purposes, but either occurred before the employee was a member in the plan, or before the plans inception. Employees have the option to purchase past service, by cash or by qualified retirement plan roll-over, to increase their years of service in the calculation of their retirement pension.
Explained further: In cases where employees are considering rolling over the assets from their qualified retirement plan, it is often wise to first consult a financial planner. Depending on a number of factors, such as employee life expectancy, marital status and quality of the pension plan, it may or may not be beneficial to make the roll-over.
Definition courtesy of Investopedia