GUIDO, your solenoid looks like those used on mowers. Call it what you want, solenoid, relay, motor contactor. Basically they all work the same way. A electo-mangetic controlled heavy duty switch.
A solenoid is an automotive term, relay is an electonic term, and a motor contactor is an air-conditioning term compressors.
A solenoid can also be a term for electro-magnetic controlled valves. Gas, air , water soleniod, ect.
Take a screw driver, touch the two large terminals and you are jumping out the switch part of the solenoid.
If this is off a mower, there are many safety switches that could be causing your problem.