principal plane

Each (rotation symmetric) lens has two principal planes. These (hypothetical) planes are perpendicular to the optical axis and are the planes on which light beams parallel to the axis coming from infinity seem to bend (and then go through the respective focal points).

The image side primary plane is formed where a light ray parallel to the optical axis enters the first lens of a lens system and intersects with the corresponding ray leaving the last lens element.

The object side primary plane is formed where a light ray parallel to the optical axis enters the last lens of a lens system and intersects with the corresponding ray leaving the first lens element.

NOTE:
This only applies to the paraxial optics, i.e. very close to the optical axis.
For rays more distant to the optical axis spherical aberration distorts this behaviour.
In a single thin lens the two principal planes merge and can be approximated by the center of the lens.