A slug is a piece of spacing material used in typesetting to space paragraphs. They are usually manufactured in strips of 6pt lead.
In modern typesetting programs such as Adobe InDesign, slugs hold printing information, customized color bar information, or displays other instructions and descriptions for other information in the document. Objects (including text frames) positioned in the slug area are printed but will disappear when the document is trimmed to its final page size. [1]
More recently this term is also used in web publishing to refer to short article labels that can be used as a part of an URL. Slugs are usually derived from article's title and are limited in length and the set of characters (to prevent percent-encoding, often only letters, numbers and hyphens are allowed).[2]