Cog/kɑːɡ/ Definition 1 of 2Pro Tip 1/3Physical Object Cog refers to the physical gears inside machines and devices. When you open the back of a watch, you can see the tiny cogs working together. Pro Tip 2/3Essential Role Each cog is crucial for the overall function, much like pieces of a puzzle. If one cog breaks, the entire mechanism could stop working. Pro Tip 3/3Mechanical Context Cog is usually used in contexts related to machinery or mechanics. The engineer explained how the cogs transfer motion within the engine.