TIP120 transistor. The transistor here has the same physical package as the voltage regulators shown above. It has three legs and a tab at the top with a hole in it. The tab is the back of the component. If you hold the component with the tab at the top and the bulging side of the component facing you, the legs will be arranged, from left to right, base, collector, emitter. The only way to know the difference between two components of the same package is to read the label on the package, unfortunately. This one is labeled TIP120.