One of the most important features of a mathematical system is that the terms used in the system must have clear definitions. When making a definition, you should use words that one simpler than the term being defined. However, if you continually try to define terms by using. Simpler words, the process could conceivably go on forever. There comes a time when a definition must use a term whose remaining is assume to be clear. Such terms are called undefined terms.
They are used to begin the process of defining new terms. We will begin our study of Euclidean geometry by considering its undefined terms : point, line and plane.
With the use of the undefined terms, we can give meanings of other terms and expressions of geometry. If we are to develop a system of geometry each proposition must logically follow from other propositions. A system of geometry must start with some propositions that are accepted as true. We simply cannot deduce all statements. These assumptions are called axioms and postulates.