A pack made up of a TP Valve and a Drain Cock.

TP Valve
Every water heater, gas or electric, should have a temperature and pressure relief valve, commonly called a T&P valve. Your water heater isn't safe without one. It's so important, in fact, that many states require factories to install T&P valves rather than leaving the decision to homeowners and installers.

Drain Cock
A drain cock is a device for venting liquid from a pressurized vessel.