What is the definition of a theorem in mathematics?

A theorem in mathematics is defined as a statement that has been proven to be true based on previously established statements, such as other theorems, axioms, and definitions. The proof is a logical argument that demonstrates the truth of the theorem, ensuring that it is not merely an assertion but a conclusion derived from accepted mathematical principles.

When considering the other options, a statement accepted without proof refers to a postulate or axiom, which is foundational in mathematics but does not encompass the nature of a theorem. A guess about a mathematical concept describes conjectures, which are ideas proposed to be true without proof, unlike the established truth of a theorem. Lastly, the definition of a mathematical term pertains to explanations or clarifications of concepts rather than the rigorous proof-based validation that characterizes a theorem. Thus, option B accurately encapsulates the essence of what a theorem is in the realm of mathematics.