From top of my head: my grandma used to have one. There was always a little flame running for safety in case of leaks, but when she used hoy water, I think the geyser just heated it on the go, instead of preheating a reservoir.
A device for heating water on-demand, usually a gas burner with a spiraling water pipe surrounding/above it. As opposed to a boiler, which pre-heats water and stores it for later use (and also needs to keep reheating the water as it cools if not used).
Could be wrong though.