A sulfide is an anion of sulfur

Sulfur

Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

 in its lowest oxidation state

Oxidation state

In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...

 of 2-. Sulfide is also a slightly archaic term for thioether

Thioether

A thioether is a functional group in organosulfur chemistry with the connectivity C-S-C as shown on right. Like many other sulfur-containing compounds, volatile thioethers have foul odors. A thioether is similar to an ether except that it contains a sulfur atom in place of the oxygen...

s, a common type of organosulfur compound that are well known for their bad odors.
The dianion S²⁻ exists only in strongly alkaline aqueous solutions. Such solutions can form by dissolution of H₂S or alkali metal salts such as Li₂S, Na₂S, and K₂S in the presence of extra hydroxide.