Ecco il w3c cosa dice in merito (fonte: specifica di html 4.0).

Definition lists vary only slightly from other types of lists in that list items consist of two parts: a term and a description. The term is given by the DT element and is restricted to inline content. The description is given with a DD element that contains block-level content.

Here is an example:
codice:
<DL>
  <DT>Dweeb
  <DD>young excitable person who may mature
    into a Nerd or Geek

  <DT>Hacker
  <DD>a clever programmer

  <DT>Nerd
  <DD>technically bright but socially inept person

</DL>
Here is an example with multiple terms and descriptions:
codice:
<DL>
   <DT>Center
   <DT>Centre
   <DD> A point equidistant from all points
              on the surface of a sphere.
   <DD> In some field sports, the player who
              holds the middle position on the field, court,
              or forward line.
</DL>
Another application of DL, for example, is for marking up dialogues, with each DT naming a speaker, and each DD containing his or her words.

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Applicandole alle barre di navigazione, è come assegnare ad una lista di link una determinata definizione.