Turing machine
A Turing machine is an abstract mathematical model of computation invented by Alan Turing in 1936. It consists of an infinitely long tape divided into cells, a read-write head that moves along the tape, and a set of rules (a "state machine") governing how the head responds to symbols it reads. Despite its simplicity, a Turing machine can theoretically compute anything that any real computer can compute—a principle known as Turing completeness.
The device was not meant to be built; rather, it served as a thought experiment to explore the boundaries of what's computable. Turing proved that some problems, like the Halting problem, are fundamentally unsolvable even by such a machine. This foundational insight shaped computer science, algorithms, and our understanding of computational limits.
The Turing machine remains relevant today in formal language theory, mathematical structures, and the study of computational complexity. It also connects historically to the Turing test, which Turing proposed as a measure of artificial intelligence—though the two concepts address entirely different questions.