The control element in the silicon controlled rectifier (SCR) is called the:

An SCR features three main terminals: the anode, the cathode, and the gate. The gate is the dedicated control element responsible for triggering the device into a conductive state. When a small positive voltage pulse is applied to the gate relative to the cathode, the SCR "fires" and begins conducting heavily between the anode and cathode. The anode and cathode are the primary current-carrying terminals, not the control elements. The distractor "emitter" is incorrect because that term belongs to bipolar junction transistors (BJTs), not thyristors like the SCR.