Imposter is also known as Test Double.
When we are writing a test in which we cannot (or choose not to) use a real depended-on component (DOC), we can replace it with a Test Double. The Test Double doesn’t have to behave exactly like the real DOC; it merely has to provide the same API as the real DOC so that the SUT thinks it is the real one!
Implementation depends on the variation:
(source: xunitpatterns.com)