modules/taxonomy/views_handler_argument_taxonomy.inc

<?php

class views_handler_argument_taxonomy extends views_handler_argument_numeric {

  function title() {
    // There might be no valid argument.
    if ($this->argument) {
      $term = taxonomy_term_load($this->argument);
      if (!empty($term)) {
        return check_plain($term->name);
      }
    }
    // TODO review text
    return t('No name');
  }
}