I've been coming accross notation like this $\gamma = (id,id)_\# \mu$, where the hashtag/pound sign is used in the subscript. From context, it seems to have something to do with marginalizing probability measures, but I can't seem to find any resource that defines this notation.
For an example of the usage, you can see page 2 of these lecture notes by Chizat.
Thanks to @FShrike'scomment, I was able to confirm that this indicates a push-forward measure. If anyone in the future is looking for a definition to reference, I found it in Exercise 1.4.38 (p.103) of Terence Tao's "An Introduction to Measure Theory": $$ \phi_\# \mu(E) = \mu(\phi^{-1}(E)). $$