In complexity theory and computability theory, an oracle machine is an abstract machine used to study decision problems. It can be visualized as a Turing machine with a black box, called an oracle, which is able to decide certain decision problems in a single operation. The problem can be of any complexity class. Even undecidable problems, like the halting problem, can be used.
Reference:
http://en.wikipedia.org/wiki/Oracle_machine
Reference:
http://en.wikipedia.org/wiki/Oracle_machine
