EM (Typography)

EM is the unit of measurement in Typography. This unit was originally derived from the width the Capital letter "M" of a particular typeface.

Originally When working with Metal Type EM was defined as the height of the metal body from which the letter rises.

This is slightly different for digital type.

"In digital type, the em is a grid of arbitrary resolution that is used as the design space of a digital font. Imaging systems, whether for screen or for print, work by scaling the em to a specified point size.

In digital type, the relationship of the height of particular letters to the em is arbitrarily set by the typeface designer. However, as a very rough guideline, an "average" font might have a cap height of 70% of the em, and anx-height of 48% of the em."

http://en.wikipedia.org/wiki/Em_(typography)