A point in 3D space.
A 3D volume in a scene positioned relative to the viewport's camera. Objects within the frustum are visible. For perspective viewing, the viewing frustum is the volume of an imaginary pyramid that is between the front clipping plane and the back clipping plane. For orthographic viewing, the viewing frustum is cuboid.
A rectangle that defines how a 3D scene is rendered into a 2D window. A viewport also defines an area on a device into which objects will be rendered.