A cog is a tooth on the rim of a wheel or gear. It can also refer to the gear itself. Cogs are an essential part of an engine, as they have tiny teeth that fit together, making the wheel turn and the engine run. The term "cog" can also be used to describe a subordinate but integral person or part of a large organization or activity. For example, someone who feels like they are just a small part of a larger system might say, "Im just a cog in the system".

In addition to its mechanical and organizational meanings, "cog" can also refer to other things. For example, "Cog" is the name of a project at the Humanoid Robotics Group at the Massachusetts Institute of Technology (MIT). It is also the name of an open source audio player for macOS.