For the purposes of this chapter “supplemental unemployment plan” shall mean a plan, system, trust or contract by the terms of which an individual will receive from the employer, trustees of the plan or trust, union or other agency, payments supplemental to unemployment compensation or based on or to be paid in conjunction with unemployment compensation, which are available to the employees generally but not available in advance, in a lump sum or for loan, to be paid only during periods of unemployment except payments for vacations, bonuses, profit sharing plans and severance pay or separation pay.