Theorem

1. Theorem is a general proposition not self-evident but proved by a chain of reasoning; a truth established by means of accepted truths.
2. A theorem is a statement in mathematics or logic that can be proved to be true by reasoning.

The proof of a theorem is a logical argument that uses the inference rules of a deductive system to establish that the theorem is a logical consequence of the axioms and previously proved theorems.
In the mainstream of mathematics, the axioms and the inference rules are commonly left implicit, and, in this case, they are almost always those of Zermelo–Fraenkel set theory with the axiom of choice, or of a less powerful theory, such as Peano arithmetic. A notable exception is Wiles's proof of Fermat's Last Theorem, which involves the Grothendieck universes whose existence requires the addition of a new axiom to the set theory. Generally, an assertion that is explicitly called a theorem is a proved result that is not an immediate consequence of other known theorems. Moreover, many authors qualify as theorems only the most important results, and use the terms lemma, proposition and corollary for less important theorems.

Sources:

Google’s English dictionary by Oxford Languages https://languages.oup.com/google-dictionary-en/

Science News For Students https://www.sciencenewsforstudents.org/article/

CorrosionPedia Dictionary https://www.corrosionpedia.com/definition/