In typography, an ascender is the portion of a minuscule letter in a Latin-derived alphabet that extends above the mean line of a font. That is, the part of a lower-case letter that is taller than the font’s x-height.
Ascenders, together with descenders, increase the legibility of words. For example, British road signs no longer use all capital letters due to ascenders.
Source: http://en.wikipedia.org/wiki/Ascender_(typography)
