<p>I think whatever the difference between <a href="https://stats.stackexchange.com/questions/tagged/definition" class="post-tag" title="show questions tagged &#39;definition&#39;" rel="tag">definition</a> (60 questions) and <a href="https://stats.stackexchange.com/questions/tagged/terminology" class="post-tag" title="show questions tagged &#39;terminology&#39;" rel="tag">terminology</a> (247 questions) might possibly be, in practice these tags are used interchangeably. Therefore I suggest hard-merging the less popular <a href="https://stats.stackexchange.com/questions/tagged/definition" class="post-tag" title="show questions tagged &#39;definition&#39;" rel="tag">definition</a> into <a href="https://stats.stackexchange.com/questions/tagged/terminology" class="post-tag" title="show questions tagged &#39;terminology&#39;" rel="tag">terminology</a>.</p>

<p><strong>Update after @Glen_b's answer:</strong> I find that tags that are subsets of other tags tend to create confusion, so I would argue this should be avoided. It seems to me that there is a lot of questions about definitions marked with <a href="https://stats.stackexchange.com/questions/tagged/terminology" class="post-tag" title="show questions tagged &#39;terminology&#39;" rel="tag">terminology</a> and not with <a href="https://stats.stackexchange.com/questions/tagged/definition" class="post-tag" title="show questions tagged &#39;definition&#39;" rel="tag">definition</a>, and so having separate sub-tag <a href="https://stats.stackexchange.com/questions/tagged/definition" class="post-tag" title="show questions tagged &#39;definition&#39;" rel="tag">definition</a> is not being very helpful.</p>

<p><strong>Update 2 after further discussion:</strong> After some further reflection, I realized that my objection to sub-tags was not as warranted as I originally thought. We actually have many sub-tags that are pretty successful, e.g. <a href="https://stats.stackexchange.com/questions/tagged/multiple-regression" class="post-tag" title="show questions tagged &#39;multiple-regression&#39;" rel="tag">multiple-regression</a> exists together with <a href="https://stats.stackexchange.com/questions/tagged/regression" class="post-tag" title="show questions tagged &#39;regression&#39;" rel="tag">regression</a> and they are both used a lot. So I must admit that my suggestion was not very well thought through.</p>
