Mean Line

Mean Line" src="http://typographydeconstructed.com/wp-content/uploads/2010/12/mean-line-white.gif" alt="Mean Line" width="190" height="190" />Imaginary line running along the top of non-ascending, lowercase letters.

wikipedia Reference

In typography, the mean line, also (and more simply) known as midline, is the line that determines where non-ascending lowercase letters terminate in a typeface. The distance between the baseline and the mean line is called the x-height.

Round glyphs will break (overshoot) the mean line slightly in many typefaces, since this is aesthetically more pleasing; a rounded shape will appear visually smaller than flat topped (or bottomed) shapes of equal height, due to an optical illusion.