在Martin Fowler的網站上看到一篇文章，題中提到一些人對於Mock或Stub之類的物件有著各式各樣的說明。Gerard Meszaros試圖替這些相關的用詞訂出明確的定義，Gerard將這些類似用途的物件統稱為Double，然後又分成以下四類：
- Dummy objects are passed around but never actually used. Usually they are just used to fill parameter lists.
- Fake objects actually have working implementations, but usually take some shortcut which makes them not suitable for production (an InMemoryDatabase is a good example).
- Stubs provide canned answers to calls made during the test, usually not responding at all to anything outside what's programmed in for the test. Stubs may also record information about calls, such as an email gateway stub that remembers the messages it 'sent', or maybe only how many messages it 'sent'.
- Mocks are pre-programmed with expectations which form a specification of the calls they are expected to receive. They can throw an exception if they receive a call they don't expect and are checked during verification to ensure they got all the calls they were expecting.