High pressure. Home improvement retailers have a pressure gauge that connects where you hook up a hose. Open the valve with no other water being used in the house and it will show your pressure in psi. (pounds per square inch). Depending on house design, you should have 45-65 psi. If you are in the 75+ range, you may need a pressure reducer installed.
If you already have a pressure reducer installed and your pressure is normal, keep the gauge in place showing the existing pressure. Still without using any water in the home, slowly turn the temperature up on your hot water tank until you hear the tank fire. For an electric hot water tank, you may not hear anything.
Watch the gauge as the water heats (maybe about 10 minutes) and see if the pressure rises. If it does, you have "thermal expansion".
(To nip it in the bud, there has been a myth accompanied by an engineering formula that states there can be no thermal expansion with the use of an electric hot water tank. My experiences with the simple test described above has shown thermal expansion to +30 psi in some cases. So please let's not go down that road.)