The above symbol is referred to as ultimate equality in Tristan Needham's Visual Differential Geometry. How do I write it in latex?
A good resource is DeTeXify; you can draw the symbol you want and it will provide possible $\LaTeX$ codes for it. Note, however, that MathJax does not support all packages, so some of the options may not be available.
For this picture, it suggests
\asymp, $\asymp$ as noted by Peter Phipps, which is regular $\LaTeX$ and so supported.