A theorem is a basic geometric principle which is supported and established by a proof.  Theorems are proven to be true by making connections between accepted definitions, postulates, mathematical operations, and previously proven theorems.    

If you have an idea that can be demonstrated to be true or works all the time, then the idea is a theorem!

Theorems are usually divided into a hypothesis (what is given) and a conclusion (what needs to be proven). Once a theorem has been proven, it becomes an accepted tool in the proof of any other theorems.