The definition should be a short nounal phrase definining the
subject of the entry. Definitions should not include commentary, implementations,
equations or formulae (unless the subject is one of these) or examples. The
description element can be used for these.
The definition can be in any markup language, but normally XHTML will be used,
perhaps with links to other XML namespaces such as CML for chemistry.
From the IUPAC Dictionary of Medicinal Chemistry
<entry id="a7" term="Allosteric enzyme">
<definition>An <a href="#e3">enzyme</a>
that contains a region to which small, regulatory molecules
("effectors") may bind in addition to and separate from the
substrate binding site and thereby affect the catalytic
activity.
</definition>
<description>On binding the effector, the catalytic activity of the
<strong>enzyme</strong> towards the substrate may be enhanced, in
which case the effector is an activator, or reduced, in which case
it is a de-activator or inhibitor.
</description>
</entry>