The TRIAC is a three terminal semiconductor device for controlling current. It gains it's name from the term TRIode for Alternating Current.
It is effectivly a development of the SCR or thyristor, but unlike the thyristor which is only able to conduct in one direction, the TRIAC is a bidirectional device.
Type: BT137F
Note: ZX-7149 is isotab - mounting tab is not connected to pins.