What was the name of the theoretical machine that can simulate any other Turing machine?

The theoretical machine that can simulate any other Turing machine was the Universal Turing machine. Alan Turing introduced this idea in his 1936 paper, “On Computable Numbers, with an Application to the Entscheidungsproblem.”

A specialized Turing machine is designed to perform one particular computation. A Universal Turing machine instead receives two pieces of information: an encoded description of another machine and that machine’s input. It then follows the encoded machine’s rules, effectively reproducing its computation.

This concept was a major step toward the modern stored-program computer, in which instructions and data can be held in the same memory. The phrase “Universal machine” can appear as a shortened description, but “Universal Turing machine” is the standard canonical name. It should not be confused with an ordinary Turing machine, which may perform only a specific task. Universality does not mean the machine can solve every problem: the halting problem remains undecidable.