Test Double (xUTP.com)

doubleには「代役」「替え玉」という意味がある（ウィズダム英和辞典）

We replace a component on which the SUT depends with a "test-specific equivalent."

Sometimes it is just plain hard to test the system under test (SUT) because it depends on other components that cannot be used in the test environment.

When we are writing a test in which we cannot (or chose not to) use a real depended-on component (DOC), we can replace it with a Test Double.

When to use itにVariation

replace a real component on which the SUT depends so that the test has a control point for the indirect inputs of the SUT

Test Spy

as an observation point that is used to verify the indirect outputs of the SUT as it is exercised.

Fake Object

Dummy Object