What is a MERIDIAN in terms of the eye?!

Lines surrounding the surface of the eyeball passing through both anterior and posterior poles.

As described for astigmatism: The refractive error of the astigmatic eye stems from a difference in degree of curvature refraction of the two different meridians (i.e., the eye has different focal points in different planes.) For example, the image may be clearly focused on the retina in the horizontal (sagittal) plane, but not in the vertical (tangential) plane. Astigmatism causes difficulties in seeing fine detail, and in some cases vertical lines (e.g., walls) may appear to the patient to be tilted.

under astigmatism in wikipedia.