A result that has been proved to be true (using operations and facts that were already known).
Example: The "Pythagoras Theorem" says that a2 + b2 = c2 for a right angled triangle. There are many proofs it is true.
Other examples:
• Intermediate Value Theorem
• Binomial Theorem
• Fundamental Theorem of Arithmetic
• Fundamental Theorem of Algebra
Lots more!
A Theorem is a major result, a minor result is called a Lemma.
See: Pythagoras Theorem