List in which each item consists of two parts: 1) a word, phrase, term, graphic, chemical structure, or equation, that is paired with 2) one or more descriptions, discussions, explanations, or definitions of it.
The following, in order:
<label> Label (of an Equation, Figure, Reference, etc.), zero or one
<term-head> Definition List: Term Head, zero or one
<def-head> Definition List: Definition Head, zero or one
<def-item> Definition List: Definition Item, zero or more
<def-list> Definition List, zero or more