Heading over the second column (the definition column) of a definition list (two-part list).
<!ELEMENT def-head (#PCDATA %def-head-elements;)* >
(#PCDATA | email | ext-link | uri | inline-supplementary-material | related-article | related-object | hr | bold | fixed-case | italic | monospace | overline | overline-start | overline-end | roman | sans-serif | sc | strike | underline | underline-start | underline-end | ruby | alternatives | inline-graphic | private-char | chem-struct | inline-formula | tex-math | mml:math | abbrev | index-term | index-term-range-end | milestone-end | milestone-start | named-content | styled-content | fn | target | xref | sub | sup | x)*
Any combination of:
...
<glossary>
<def-list>
<title>ABBREVIATIONS</title>
<term-head>Abbreviation</term-head>
<def-head>Expansion</def-head>
<def-item><term id="G1">PAP I</term>
<def><p>poly(A)polymerase I</p></def>
</def-item>
<def-item><term id="G2">PNPase</term>
<def><p>polynucleotide phosphorylase</p></def>
</def-item>
<def-item>...</def-item>
...
</def-list>
</glossary>
...