An undefined term in geometry means that such terms cannot be explained properly. The examples of undefined terms in geometry are point, line, and plane. A point is a dimensionless term, which has no length or width or thickness. Similarly a line or a plane also has no width. A line can be extended in two directions, while a plane can be extended in all directions.
On the other hand undefined terms combines in a meaningful way, i.e. when a line crosses another lines, it forms an angle. Thus angle is a defined term in geometry.