If it's more cylindrical shape, with two big terminals on opposite sides plus a couple thin wires coming out, it's the main disconnect solenoid switch--the thing that actually turns on and off when you press the battery disconnect switch on or off. If you don't have a battery disconnect switch, then you presumably don't have one of these latching relays either.
The auto-resetting circuit breakers like Old-Biscuit posted are quite common, and come in a variety of different ratings (up to 50A). There are also, rather less commonly, manual reset ones that look quite similar but have a little reset tab that sticks out one end when they trip.