A test based on the concept
that if your theory conflicts with reality, it must be in error.
As an act: To test your theory, assumptions, plan or program against reality by seeing if gives sensible results in known situations. For e.g, to plug in two plus two and verify that you get four.
As an event: To have to abandon your theory when reality fails to honour it. For e.g. When testing the theory that you can walk through wall, to bang your head.
As a process: A safeguard designed to prevent wilder flights of fancy.
The source of many bad metaphors such as My reality check bounced.