phosphorus hep·ta·sulfide \-|heptə+\noun Etymology:heptasulfide from hepta- + sulfide : a yellow crystalline compound P4S7 made by heating a mixture of phosphorus and sulfur in a ratio of about 4 to 7 equivalents, regarded formerly as a trisulfide, and used chiefly in the synthesis of organic sulfur compounds (as thiophene); tetra-phosphorus heptasulfide