I think the most difficult word to define in geometry is point.
Other words like line, segment, circle, angle may be defined from other word based of the notion of point.
But point is a very abstract notion, because it does not have length, so a point is an imaginary think.
Once, you have the notion of point, you can figure out that a line is an infinite succession of points, and from that define other concepts.
Angle may also be found a dificcult word to define because it is the opening or amount of turn between two lines that have a common end point.
