An abstract form of a computing device that is more like a software program than a piece of hardware. Any so-called Turing machine can be implemented on an infinite number of computing devices. A Turing machine would have a read/write head that scans a one-dimensional, bidirectional tape divided into equal-sized sections inscribed with a 1 or a 0. Computation starts when the mechanism in a given “state” scans a section, erasing what it discovers there, printing a 0 or a 1, moving to an adjacent section, and going into a new state.

This behavior is determined by three key parameters: (1) the state the mechanism is in; (2) the value in the section the mechanism is scanning; and (3) a set of instructions. For decades, a number of computer scientists have proven that if conventional digital computers are considered in isolation from random external inputs (for example, a stream of bits produced by radioactive decay), then with enough time and tape, a Turing machine could calculate any function a digital computer could calculate.

See Also: Computer.