It’s carbonated water.
Everything you ever wanted to know about water as a mixer can be found here.
I think club soda has sodium bicarbonate dissolved in the water, whereas “seltzer” often means water with carbon dioxide dissolved in it. “Soda” in chemistry generally refers to sodium, so for instance “caustic soda” is sodium hydroxide. But “seltzer” means different things to different people. It can mean flavoring and even sugar are added, in some cases. I drink carbonated water in restaurants and bars whenever it’s available, and it is often hard to express the idea clearly. Also, in restaurants and especially bars, if they have one of those dispensing machines with the buttons on the delivery end of a hose, the one for “soda water” usually adds carbonation but no sodium.
Thank You!
That website is wrong about at least one thing. Sparkling mineral water is seldom, if ever, carbonated naturally by absorbing carbonates from the soil. Perier claimed for many years to sell naturally cabonated mineral water, untill it was revealed (about ten years ago, I think) that this is not the case. (They do obtain natural CO2 from a source in the ground, but they then add it artificially to water obtained from a nearby natural spring.) I don’t know of any mineral water that is truely naturally carbonated, and certainly most of it isn’t.