Three undefined terms in geometry are
1 - Point
A point has no measurement or genuine size. A point is usually named with a capital letter. In the coordinate plane, a point is represented by (x,y).
2 - Line
A line has no thickness and its length extends in one measurement and goes on in both directions forever. A line is named by a single lowercase letter or by any two points on the line.
3 - Plane
A plane has no thickness and it extends indefinitely in all directions. A plane is named by a single letter (plane x) or by three non-collinear points (plane XYZ).