Soda water (also called club soda, or sparkling water when unflavored) is plain water carbonated under pressure, giving it a crisp, slightly acidic fizz from the carbonic acid the dissolved carbon dioxide forms. It can be made commercially or at home with a carbonation system.
It is the standard mixer for highball cocktails like the gin and tonic and vodka soda, and in Italy it is commonly paired with aperitifs like Campari or Aperol. It also serves as a base for homemade sodas, mocktails, and fruit-juice spritzers.
Soda water is distinct from tonic water, which is also carbonated but sweetened and bitter with quinine, and from seltzer, which is plain carbonated water with no added minerals - club soda typically has added minerals for a slightly different taste.