What are points that lie in the same plane?

Points that lie in the same plane are called coplanar points. Points that do not lie in the same plane are non-coplanar. Two points are always coplanar, but three or more points may or may not be coplanar.

The vertices of a triangle are a good example of three coplanar points. Similarly, a square, rhombus, trapezium and rectangle are two-dimensional plane figures. All four vertices of these figures are coplanar points.

A cylinder is a three-dimensional figure. The points on the curved surface and the points on the bottom of the cylinder do not lie in the same plane and thus are non-coplanar.