In typesetting, a slug is a piece of lead or other type metal, in any of several specific word senses. In one sense, a slug is a piece of spacing material used to space paragraphs. In the era of commercial typesetting in metal type, they were usually manufactured in strips of 6-point lead. In another sense, a slug is one line of Linotype typeset matter, where each line corresponds to one piece of 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. 
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, often only letters, numbers and hyphens are allowed).
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