
Tangential and sagittal planes
The tangential plane (also called the meridional plane) is the plane that contains the chief ray and the symmetry axis (z-axis). The sagittal plane is the plane that contains the chief ray and is orthogonal to the tangential plane. For the case that the imaged point is on the y axis, the tangential plane is the yz plane. In this case the picture is the following:

The tangential plane is the plane of a planar optical system. Strictly speaking, the sagittal plane varies whenever the chief ray is refracted.