A TRIAC is:

A TRIAC, or Triode for Alternating Current, is a type of semiconductor device that acts like a switch, controlling power to an electrical load. It is used in applications where control of alternating current is required. The comparison to a "three-way" diode relates to how a TRIAC can conduct current in both directions, unlike a standard diode, which only allows current to flow in one direction. This characteristic makes TRIACs particularly useful in applications such as light dimmers, motor speed controls, and other circuits that need to manage AC power efficiently.

In contrast, the other options describe components that do not share the same properties or functional characteristics as a TRIAC. Recognizing the specific traits of a TRIAC, especially its ability to control AC current flow in both directions, is fundamental for understanding its application and comparison to diodes.