surface & plane

The surface is the outside part or top layer of something.

eg The surface of the Moon is rocky, but the surface of the Earth is about 70% water.

A cube has 6 faces (sides). Each face is a surface of the cube.

A plane is a flat surface that extends forever.

eg In 3-dimenions (3D), the x and y axis create a plane (flat surface that goes on forever).