Undefined terms, defined terms, and postulates will help you prove Theorems. An undefined term is a term that is not easy to be defined. There are terms that really haven't a definition. Defined term is a term that has some sort of definition or has a simple definition. In geometry, we use an undefined term to define a term. Postulate are the facts that are accepted without a proof. However, if we use a little bit of critical thinking, we can use undefined and defined terms to help support a postulate. A theorem is completely opposite of a postulate and it can be proven. If the statement can be proven then this statement is a theorem.