A tangential (also called meridional) plane in 3D is in general a plane containing the optical axis.
The tangential plane through a point is the plane in 3D that contains the point and the optical axis.
In the following interactive graphics you can move the green point (click to change directions from x-y to z). The rays leave the point direction entrance pupille and ‘fill’ it completely.
Perpendicular to it is the sagittal plane.