What Is Theorem?

Theorems is what mathematics consists of, isn't it? We may learn the etymology of the word from The Words of Mathematics by S. Schwartzman:

A theorem is a proven mathematical statement, although, as an exception, some statements (notably Fermat's Last Theorem, or FLT) have been traditionally called theorems even before their proofs have been found. To prove a statement means to derive it from axioms and other theorems by means of logic rules, like modus ponens. A proof is needed to establish a mathematical statement. A single counterexample suffices to refute such a statement. (Say, in a right triangle with hypotenuse c and legs a and b, the inradius r = (a + b - c)/2. However, this is not true that for any triangle with sides a, b, c, where c is the longest, the inradius r = (a + b - c)/2. To see why it is not so, suffice it to observe that the inradius of an equilateral triangle with side a equals √3/6.

There is certainly an ambiguity mathematicians live with. Some rules (like the Law of Excluded Middle) and some axioms (like the Axiom of Choice) are not universally accepted by all mathematicians.

Commonly, auxiliary theorems of a lesser significance are called lemmas. If there is a need to emphasize an importance of a theorem in proving another theorem, the latter if called a corollary from the former, especially when the proof at hand is short.

References

1. E. J. Borowski & J. M. Borwein, The Harper Collins Dictionary of Mathematics, Harper Perennial, 1991
2. J. Daintith, R. D. Nelson (eds), The Penguin Dictionary of Mathematics, Penguin Books, 1989
3. J. Mason, Thinking Mathematically, Addison-Wesley, 1985
4. G. Polya, How To Solve It, Princeton University Press, 2nd ed, 1957
5. S. Schwartzman, The Words of Mathematics, MAA, 1994