RSpec Mocks 中是这样描述的，但是没看懂是什么意思，和具体用法。。。
Test Doubles A Test Double is an object that stands in for a real object in a test. RSpec creates test doubles that support method stubs and message expectations.
book = double("book")
stub are aliases of the more generic
describe, they can be used interchangeably to make the intent of the specs more clear. This is described in a lot more detail in The RSpec Book.
rspec-mocks helps to control the context in a code example by letting you set known return values, fake implementations of methods, and even expectations that specific messages are received by an object.
You can also use the mock and stub methods to create test doubles, however these methods are there for backward compatibility only and will likely be deprecated and then removed from future versions.
所以 double 和 stub 、 mock 其实都是用来生成 mock 对象的。