Undefined Terms

In geometry the words, Point, Line and Plane are undefined terms. That means that these words do not have a definition, but there is an agreement about what they mean.

Point

A point has NO dimension and is represented by a dot.

Point B, E, C, D are all points on this plane

Line

A line has ONE dimension and is represented by a line with two arrowheads. It extends infinitely in each direction.

Through any TWO points there is exactly ONE line.

You can use any TWO points to name a line.

Line AB, Line CD, Line BE

Plane

A plane has TWO dimensions and is represented by a shape that looks like a floor or a wall.

Through any three points not on the same line, there is exactly one plane. You can use three points that are not all on the same line to name a plane. (non-coplanar)