A letter’s relative amount of blackness. Proper terminology for weight has never been precisely determined. In types used for continuous reading, two weights are generally used the original design, called either regular or light, and a boldface. Square serif and sans serif types have as many as eight or nine different weights, differently described by each manufacturer. Most likely this imprecision can never be corrected.
See weights names on Style variation scheme?>
(1) The width of a line. Type is also measured in width, or set size. A line of type is measured in ems. An em is equal to the square of the type body. It was originally so called because the type body bearing a letter m is square. For example, a pica em is 12 points wide. A space half as wide as the em is called an en. The length of line required to set the alphabet of small, or lowercase, pica letters is 13 ems. If this alphabet takes more than 13 ems, it is said to be a fat or expanded face. If it takes less space, it is said to be lean or condensed.
See example of widths on Style variation scheme?>
(2) The width of a letter, from the left edge to the right edge of its contour.
The space between words. When type is set FL/RR, the word space may be of fixed size, but when the type is justified, the word space must be elastic.