A compound of sulfur with a metal or with another element — sulfur combined with something else, where the sulfur has not been joined to oxygen. The one everybody has met without knowing its name is hydrogen sulfide, the gas that smells of rotten eggs; it is what you smell near hot springs, drains and mudflats, and it is genuinely dangerous in quantity, which is worth remembering because at high concentrations it stops smelling and people think the danger has passed. Others turn up in mining and geology, because many metal ores are sulfides — iron sulfide (fool's gold is one form), lead sulfide, zinc sulfide. This f spelling is the American one; British writing traditionally uses sulphide. As with sulfate, the international chemistry body settled on the f, so science writing everywhere now uses sulfide. Keep the ending straight: -ide means sulfur joined directly to another element, -ate means a salt of sulfuric acid, with oxygen in it. Different endings, different compounds, and the exam will ask.