As per the title, definition's usage guidance is "for questions that seek to clarify the meaning of a term." As this is LawSE it seems to me that the only terms that require defining are legal ones.
should the "definition" and "legal-terms" tags be merged?
No. Users might need help understanding the relevance or scope of a word in statutes or clauses, but that does not necessarily render that specific word an instance of legal terminology.
Likewise, courts oftentimes resort to the commonplace meaning of crucial words when justifying their interpretation of some text(s) relevant to a controversy. When doing so, they typically quote definitions from sources other than legislation or legal dictionaries.