In typesetting, a set of numbers defining the amount of space that should be allotted to each character in a particular font.
Newer versions of professional software allow the user to choose a preferred method of letter-spacing, either "metrics" or "optical." When fonts lack thorough metrics tables (many so-called "free" fonts), "optical" usually yields better-looking spacing.
Kerning and kerning pairs are related concepts. See also Adobe font metrics.