A rich hypertext facility used in SGML (StandardGeneralizedMarkupLanguage) documents. XLink is to HyTime as XML is to SGML.

From http://www.oasis-open.org/cover/hytimeKimber9701.html :

"HyTime is an application of SGML: to oversimplify, HyTime defines a set of
element types that you can use with your existing SGML document types to
provide hyperlinking and other facilities in a standardized way.

In other words, HyTime effectively extends SGML by defining a standard set
of facilities for doing hypertext and multimedia presentations.  

HyTime does not compete with or change SGML in any way--it simply builds on
the basic facilities SGML provides.  In particular, HyTime provides the
following hypertext-related facilities:

	1. A general model of hyperlinks and syntax for hyperlink representation using SGML elements.  This is much more than simple ID/IDREF.
	2. Facilities for addressing things other than elements with IDs in the local document: cross-document ID refs, references to elements without IDs, references to data characters,references to multiple objects at once.

These are facilities that all hypertext applications need.  HyTime provides
a general and robust mechanism for adding them to SGML documents and SGML
processing applications.  HyTime can also be integrated with other
mechanisms, such as TEI extended pointers or HTTP URLs."

See also: http://www.oasis-open.org/cover/hytime.html