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A Turing Machine which, by appropriate programming using a finite length of input tape, can act as any Turing Machine whatsoever.
See Chaitin's Constant, Halting Problem, Turing Machine
References
Penrose, R.  The Emperor's New Mind: Concerning Computers, Minds, and the Laws of Physics.
  Oxford: Oxford University Press, pp. 51-57, 1989.