I had to look it up to see what it's called. Evidently it's called a "sweat bag". Google and the online dictionaries haven't caught up yet as of this writing (they still call it "a bag that holds water and cools it by evaporation"). But the emerging definition is that it's a bag that:
1) Seals in moisture to protect the rest of your luggage from getting wet from sweat or pool water.
2) Seals in odor to let you make the trip from the gym home without causing Pepe LePew-like reactions
3) Has anti-microbial qualities to prevent bacterial growth (and further stinking)
4) Is lightweight and fits into a gym bag or luggage
5) Is machine washable
This brand called Ornadi seems to check all the boxes, and is made in the USA.
The company that seems to have popularized these is called The Sweat Mate; their bags seem to have everything except #3. On their Web site, they do say that even though their fabric is manufactured "overseas" (we all know what that means) they do all their manufacturing in the US.
As you can imagine, there is a flood of imitators from China that look nice on the outside but don't even come close to performing as well. And yet thousands of Americans will buy them.
Be sure to support these bona fide American companies to make sure they stay in business.