I think whatever the difference between definition (60 questions) and terminology (247 questions) might possibly be, in practice these tags are used interchangeably. Therefore I suggest hard-merging the less popular definition into terminology.
Update after @Glen_b's answer: 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 terminology and not with definition, and so having separate sub-tag definition is not being very helpful.
Update 2 after further discussion: 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. multiple-regression exists together with regression and they are both used a lot. So I must admit that my suggestion was not very well thought through.