Three undefined terms in geometry are point, line and plane. These three terms are explained but not defined as everyone has an intuitive idea of these concepts. These terms serve as the foundation on which geometry is built.
Space is the biggest set in geometry. All other sets are subsets of space. Points, lines, planes, angles and rays are all subsets of space. A point is dimensionless and hence cannot be physically represented on paper. However, for the purpose of the study of geometry it is represented by a dot on paper.
A line does not have width, and it is infinite in length. Again, this makes it impossible to represent an entire line on paper. A plane has no thickness and no boundaries. It stretches indefinitely in all directions. It is represented on paper as a four sided closed figure.