Currently there are two tags that cover roughly the same territory: definitions and assignment. The question is which should be used? From a user perspective, I think definitions is better, but I'd like the community's input.
I vote for definitions as well for the same reasons as Szabolcs. However, I wonder how useful either tag is. I'm new to this but based on other posts I've read tags are supposed to make it easier to find a question.
When might a user be interested in all questions tagged definitions? I'm sure I'm missing something but this doesn't sound like a useful search criterion to me.
Edit: Here are some possible suggestions for the questions currently tagged with definitions.
How to check for Mathematica’s definition of XY? implementation This question is asking what the underlying algorithm is doing. I'm not in love with implementation but I think we need something more specific than definitions unless this is the only way we use it.
The various *values are things lots of users have questions about. I think OwnValues, DownValues, SubValues, and UpValues all deserve their own tag (or perhaps a tag that refers to all of them in a clear way).
I vote for assignment. In a mostly immutable system like Mathematica, operations that change the global state are very important. The word "assignment" reflects the process of changing state much better than "definitions". Besides, "definitions" is really unclear for non-experts: do we mean values for symbols, or do we mean something else. Even for experts, this word does not have a single well-defined meaning. It may be that we'll still need something like this, but IMO assignment we need more, and we can also do something along the lines proposed by @Andy - use several more focused tags.
So there might be room for both, but assignment works better for me and is less ambiguous. I am not sure the distinction Szabolcs makes is one that will be made by less experienced users and people who know other programming languages. We don't want to make more retagging work for ourselves.
I don't favor tags for UpValues, DownValues etc separately. These will be visible to a search anyway, and unlikely to be followed as separate tags.
I vote for definitions:
When I hear 'assignment', I think of variable assignment, i.e. only
When I hear 'definition', I would also include
DownValues. So 'definition' is more general than 'assignment'.
I don't know if others use the terms in the same way or not though ...