Proposal 1: (Less distinction) Merge [terminology] with [definition]
Here is my interpretation of the three tags:
[notation] contains questions about how symbols, functions or operators are written, and their conventions.
[definition] holds questions about the meanings of mathematical terms.
[terminology] has questions about A) usage, history, and the "why" of terms; and B) questions seeking names for mathematical phenomena.
I believe that the current status quo is confusing because the A) part of [terminology] comes close to [definition], and it does not seem easy for a user to differentiate when asking questions about meanings and usage. Additionally, [terminology] seems too vague a term for how the community is using it. To me, [definition] would naturally be a subset of [terminology].
The English.SE site has finer distinctions than we do, using multiple tags such as [meaning], [names], [etymology] and [word-usage]. But then again, words are their bread and butter, while the proportion of questions about words on Math.SE is smaller.
It comes down to what distinctions the community wants. Here are some ideas I have: