An HTML tag for a definition list.

In HTML 3.2, the tag is defined as
<!ELEMENT DL    - -  (DT|DD)+>

Which means that the tag may only contain DT and DD elements, in any order. That's right: you can have DT DD DD DD DT DT and it is fine and dandy. And you need at least one DT or DD.

The definition remains unchanged in HTML 4.0, whose spec provides the following helpful examples:

young excitable person who may mature into a Nerd or Geek
hacker on the Internet
male so into the Net that he forgets his wife's birthday
A point equidistant from all points on the surface of a sphere.
In some field sports, the player who holds the middle position on the field, court, or forward line.

It is of historical interest to notice that in HTML 2.0 it was said that:

The content of a DL element is a sequence of DT elements and/or DD elements, usually in pairs. Multiple DT may be paired with a single DD element. Documents should not contain multiple consecutive DD elements.
(italics mine). The most recent definition is more permissive.