1. （嵌）齿轮
A cog is a wheel with square or triangular teeth around the edge, which is used in a machine to turn another wheel or part.

2. (大型机构或团体中的)一小分子
If you describe someone as a cog in a machine or wheel, you mean that they are a small part of a large organization or group.

1. tooth on the rim of gear wheel

Synonym: sprocket

2. a subordinate who performs an important but routine function

e.g. he was a small cog in a large machine