My opinion is that if you are going to ask a question, then it is in the best interest of everybody, with no exception, to include as much information as you (strictly) need in order to make yourself understood.
According to this philosophy, I would recommend that it is not only a very good idea, but should be mandatory to make the effort to explain (in a few words, that's all) what your notation is.
I made a comment on a post recently (which no longer exists, it was deleted as a duplicate question) that the notations $\Bbb F_2$ and $\langle {x^3 + x^2 + 1}\rangle$ (or some such) needed to be explained in the body of the question. However, I received a rebuke in the comments that it is not necessary to define them, because if you don't immediately know what they are from knowledge of the context in which the question is being asked (that context, in this case, being apparent from the fact that the word "field" appeared in the title), then you won't understand what they mean even if this information was, and that it is sufficient for them to be explained in a comment.
In short, if you don't understand what the post is about, you don't get to be told.
Fair enough, you need to know your knowledge domain before you can answer it, but I would have thought that MSE was more than just a question-and-answer forum. Ideally it would be a place where less capable mathematicians and students of mathematics can come to learn something new, and to be able to read an existing question and answer and pick up on an area of maths which they aren't familiar with.
What do others think the "purpose" of MSE actually is? To answer questions posed as pithily and efficiently as possible, or to contribute towards the spread of knowledge and skill in general?