a part of a machine, or a set of parts that work together

These automatic cameras have a special focusing mechanism. 这些自动相机有特殊的对焦装置。

a way of doing something that is planned or part of a system

The mechanism for collecting taxes needs revising. 收税制度需要修改。

a part of your behaviour that helps you to deal with a difficult situation

She's actually kind of insecure, and her rudeness is just a defence mechanism. 她其实很缺乏安全感，她的粗鲁不过是出于防卫心理。