A chemical compound — a salt or ester of sulfuric acid; that is, what you get when sulfuric acid reacts with something else and gives up its hydrogen. Common ones a student may actually meet: copper sulfate (the bright blue crystals in school science), magnesium sulfate (Epsom salts, used in baths), calcium sulfate (plaster). This spelling with an f is the American one; British writing uses sulphate. The two are the same word, said the same way, and the difference is only which side of the Atlantic — though it is worth knowing that the international body of chemists (IUPAC) chose the f spelling as the official one, so scientific journals worldwide, British ones included, now write sulfate. That means the 'American spelling' label is true for ordinary writing but not really for science any more. You will also see it outside the lab: shampoo bottles say 'sulfate-free', meaning without the strong detergents (like sodium lauryl sulfate) that make foam and can dry out hair. There is also a rare technical verb, to sulfate, used about a car battery going bad as sulfate builds up on its plates.