A slug is a piece of spacing material used in typesetting

Typesetting

Typesetting is the composition of text by means of types.Typesetting requires the prior process of designing a font and storing it in some manner...

 to space paragraphs. They are usually manufactured in strips of 6pt lead.

In modern typesetting programs such as Adobe InDesign

Adobe InDesign

Adobe InDesign is a software application produced by Adobe Systems. It can be used to create works such as posters, flyers, brochures, magazines, newspapers and books. In conjunction with Adobe Digital Publishing Suite InDesign can publish content suitable for tablet devices...

, 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.

Usage in web publishing

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

Percent-encoding

Percent-encoding, also known as URL encoding, is a mechanism for encoding information in a Uniform Resource Identifier under certain circumstances. Although it is known as URL encoding it is, in fact, used more generally within the main Uniform Resource Identifier set, which includes both Uniform...

, often only letters, numbers and hyphens are allowed).