Replace Taxomony Description Field with Visual/WYSIWYG Editor

For those still looking for a solution to this problem, it's worth mentioning that the Yoast SEO plugin adds a WYSIWYG editor for descriptions automatically on the taxonomy term edit screen.

If you're not planning to use that plugin for SEO please don't add it only for this functionality. Given the plugin's popularity, I figured it was worth noting it here since it could save time for someone who already plans to install the plugin.

You can use a {$taxonomy}_edit_form_fields action hook to add html to the term edit table. In that HTML you can remove description textarea and add tinymce editor

add_action("{$taxonomy}_edit_form_fields", 'add_form_fields_example', 10, 2);

function add_form_fields_example($term, $taxonomy){
    ?>
    <tr valign="top">
        <th scope="row">Description</th>
        <td>
            <?php wp_editor(html_entity_decode($term->description), 'description', array('media_buttons' => false)); ?>
            <script>
                jQuery(window).ready(function(){
                    jQuery('label[for=description]').parent().parent().remove();
                });
            </script>
        </td>
    </tr>
    <?php
}