What is a Turing Machine?

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The formal definition of a Turing Machine is the 7-tuple:

where

These components of the 7-tuple adhere to the following rules:

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If you simply Google Formal definition of a Turing Machine, you'll get a large selection of sites where you'll see something similar to what's above on this page. They're not all the same (in fact, you may find a definition of a Turing Machine as a 6-tuple), but they're all similar.

If you go to any bookstore website and seach for "Theory of Automata", you may find a nice selection of books to choose. Most of them will probably give a formal definition of a Turing Machine as a 7-tuple, similar to what's above on this web page.

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The formal definition of a Turing Machine that's on this web page closely follows that found on page 319 in the book Introduction to Automata Theory, Languages, and Computation by John E. Hopcroft, Rajeev Motwani, and Jeffrey D. Ullman (Addison-Wesley, 2001, 2nd Edition).