What term did Alan Turing originally use for a Turing machine?

Alan Turing originally used the term “automatic machine” for what became known as a Turing machine.

In his 1936 paper “On Computable Numbers, with an Application to the Entscheidungsproblem,” Turing described an abstract device whose next action was completely determined by its current configuration. He abbreviated this deterministic model as an “a-machine,” short for automatic machine. It was a mathematical model, not a practical machine built from hardware.

The familiar expression “Turing machine” was applied later. Turing’s doctoral adviser, Alonzo Church, used that name in a review of the paper, and it eventually became standard in mathematics and computer science. Turing also distinguished automatic machines from “choice machines,” in which an external choice could influence the next step.

A common mix-up is confusing this theoretical device with Turing’s later engineering work, including the ACE, or Automatic Computing Engine. A Turing machine uses an idealized tape, symbols, a read-write head and a finite set of rules to model computation and demonstrate limits such as the undecidability of the halting problem.