get the term by its slug and echo out the descriptions
elseif {is_tax ( 'product_tag' ) ){
$term_slug = get_query_var( 'term' );
$taxonomyName = get_query_var( 'taxonomy' );
$current_term = get_term_by( 'slug', $term_slug, $taxonomyName );
?><h1 class="entry-title"><?php echo $current_term->description; ?></h1> <?php
}