The mathematica tag is for questions relating to the Mathematica software. People (typically newcomers) sometimes mistakenly add this tag to questions when they really mean math, not Mathematica. This misunderstanding is very common, and not surprising, given that typing "mat..." causes the autocompletion to suggest mathematica and that "mathematica" is the term used for math in many languages.
Update: It seems people are in favour of just renaming the tag to
wolfram-mathematica and not having any synonym for it, so I'm modifying the proposal to suggest this solution (i.e. no synonym). If you disagree, please comment.