noun
1.
a. : an automatic escape or relief valve (as for a steam boiler or hydraulic system) held by an arrangement exerting a definite usually adjustable pressure so that the valve lift and the steam, water, or other contents escape when the pressure exceeds a predetermined amount
b. : a similar valve opening inward to admit air to a vessel in which the pressure is less than that of the atmosphere and prevent collapse
2. : something that serves as an outlet for an excess or pressure
wit is the best safety valve modern man has evolved — A.A.Brill
emigration can serve as a safety valve for transitory problems of critical population pressure — President's Commission on Immigration & Naturalization