<p>I don't think these should be synonyms:</p>
<ul>
<li><p><strong>Validity</strong> is a <em>property</em> of something.  For example, it can be a <a href="http://en.wikipedia.org/wiki/Validity_%28statistics%29" rel="nofollow noreferrer">property of a measurement</a>--that it measures what it purports to measure.</p>
<p>This is what the tag wiki excerpt currently indicates, and that seems to be how it's being used (<em>note: I edited one question where</em> <code>validity</code> <em>was being used in a different sense</em>).  Nonetheless, there are other possibilities, such as a property of an <em>experiment</em> (e.g., internal &amp; external validity) or a property of a <em>model</em>; if need be, we could create additional tags to denote these and ensure the excerpts do a good job of guiding people in their use.  At present, this doesn't seem to be necessary, though.</p>
</li>
<li><p><strong>Validation</strong> is a <em>process</em>.  Examples might be <a href="https://stats.stackexchange.com/tags/cross-validation/info">cross-validation</a> or <a href="https://stats.stackexchange.com/tags/sensitivity-analysis/info">sensitivity analyses</a>.  (I'm sure there are more.)</p>
</li>
</ul>
<p>However, I do think we need to add a tag wiki excerpt for <a href="https://stats.stackexchange.com/questions/tagged/validation" class="post-tag" title="show questions tagged &#39;validation&#39;" rel="tag">validation</a> to guide people in its appropriate use.  We simply need to figure out here what that should be.</p>
