$element['description_term'] = array(
      '#type' => 'entity_autocomplete',
      '#target_type' => 'taxonomy_term',
      '#selection_settings' => [
        'target_bundles' => [
          'descriptors'
        ],
      ],
      '#title' => t('descriptor'),
      'default_value' => previously-entered-value,
      // this didn't work... '#default_value' => isset($items[$delta]->descriptor) ? $items[$delta]->descriptor : NULL,
      '#required' => FALSE,
  );