How do you theme the taxonomy description field?
The term description isn't a field, it's actually a property of the taxonomy term entity and doesn't have a theme/template associated with it.

It's built up in taxonomy_term_view() as a standard render array:

$build['description'] = array(
  '#markup' =&gt; check_markup($term-&gt;description, $term-&gt;format, '', TRUE),
  '#weight' =&gt; 0,
  '#prefix' =&gt; '&lt;div class="taxonomy-term-description"&gt;',
  '#suffix' =&gt; '&lt;/div&gt;',
);


So to theme it you could write a preprocess function for the taxonomy term template, maybe something like:

function MYTHEME_preprocess_taxonomy_term(&amp;$vars) {
  if (!empty($vars['content']['description'])) {
    $vars['content']['description']['#prefix'] = '&lt;div class="my-class"&gt;';
    $vars['content']['description']['#suffix'] = '&lt;/div&gt;';
  }
}