The collector is the terminal in a bipolar junction transistor (BJT) that collects the charge carriers injected by the emitter and controlled by the base. In an NPN transistor, it is the positively biased terminal where the amplified output current typically flows out of the device. It is physically the largest region of the transistor to help dissipate the heat generated by the power it handles.
Heard on the job
Measure the voltage drop between the collector and the emitter to see if the transistor is fully saturated.