If you really want a formal definition, you could read this: https://profsandhu.com/cs6393_s14/popek-goldberg-1974.pdf https://profsandhu.com/cs6393_s14/popek-

If you really want a formal definition, you could read this: https://profsandhu.com/cs6393_s14/popek-goldberg-1974.pdf https://profsandhu.com/cs6393_s14/popek-goldberg-1974.pdf Though some details may arguably be outdated, the general concept applies.