Kerning
In typography, kerning (less commonly mortising) is the process of adjusting the spacing between characters in a proportional font, usually to achieve a visually pleasing result. Kerning adjusts the space between individual letter forms, while tracking (letter-spacing) adjusts spacing uniformly over a range of characters. In a well-kerned font, the two-dimensional blank spaces between each pair of characters all have a visually similar area.
The related term kern denotes a part of a type letter that overhangs the edge of the type block.
Metal typesetting
The source of the word kern is from the French word carne, meaning "projecting angle, quill of a pen". The French term originated from the Latin cardo, cardinis, meaning "hinge". In the days when all type was cast metal, parts of a sort that needed to overlap adjacent letters simply hung off the edge of the sort slug. The bit of metal that hung over the edge was called a kern. At that time, the word "kerning" only referred to manufacturing the sorts with kerns, while adjusting space between letters during compositing was called inter-spacing or letter spacing.