<p>In addition to @chl's good answer, it's worth noting that such a tag would be a (so-called) 'meta tag'.  (A <em>meta tag</em> is a tag about a property of the question, rather than categorizing its content; the only meta tag left is <a href="https://stats.stackexchange.com/questions/tagged/homework" class="post-tag" title="show questions tagged &#39;homework&#39;" rel="tag">homework</a>.)  Stack Exchange explicitly discourages meta tags.  You can read the official statement here: <a href="http://blog.stackoverflow.com/2010/08/the-death-of-meta-tags/">The death of meta tags</a>, and about it's adoption on CV here: <a href="https://stats.meta.stackexchange.com/questions/264/">the-final-death-of-meta-tags</a>.  </p>
