As far as I can tell, both mathjax and tex are used here on meta for questions about usage of MathJax in posts on the main site. (AFAICT MathJax is the correct terminology, but unsurprisingly many people know this syntax simply as TeX/LaTeX.) At the moment there are 66 question tagged (mathjax), 31 questions tagged (tex) and 26 questions have both tags.
It is not ideal to have two separate tags for the same topic - for example, it complicates things when searching using tags. So I think these two tags should be synonyms.
I've checked a few other sites that use MathJax. On some other metas:
- On Mathematics, (mathjax) has synonyms (tex) and (latex), with (mathjax) as the master tag. Similarly on Computer Science.
- On Meta Stack Exchange, there is only mathjax tag. Similarly on Chemistry, MathOverflow.
- Electrical Engineering has three separate tags (tex), (latex) and (mathjax).