As comments have already mentioned, on SE sites you will either see MathJax's default
MathJax_Math font or the STIX fonts (if you have a local copy of the regular "STIX-general" fonts; e.g., OSX ships with those).
For typesetting math in Microsoft Word, it's best to use Word's native facilities (rather than external editors) since Word's implementation actually follows Appendix G of the TeXbook (like MathJax btw). For this to work, its best to use fonts with OpenType MATH Tables; unfortunately, there are very few of those around but in this case there are obvious options:
- The Latin Modern fonts work with Word and are quite similar to
MathJax_Math since both are derived from Knuth's Computer Modern.
- The STIX fonts also have a Word-compatible release, STIX word.