Zung Sung Enterprise Co., Ltd.

A starter solenoid  is the part of an automobile which switches a large electric current  to the starter motor in response to a small control current, and which in turn sets the engine in motion. 

Its function is thus identical to that of a transistor, but using an electromagnetic solenoid  rather than semiconductor to perform the switching. 

In many vehicles the solenoid also engages the starter pinion with the ring gear of the engine.

It also called “Starter Solenoid Relay Switch” or “Solenoid Relay Switch”.