"melange (me-lange also ma,lanj) n-s, original uncertain (thought to derive from ancient Terran Franzh): a. mixture of spices; b. spice of Arrakis (Dune) with geriatric properties first noted by Yanshuph Ashkoko, royal chemist in reign of Shakkad the Wise; Arrakeen melange, found only in deepest desert sands of Arrakis, linked to prophetic visions of Paul Muad'Dib (Atreides), first Fremen Mahdi; also employed by Spacing Guild Navigators and the Bene Gesserit." ―Dictionary Royal, fifth edition
The Spice Melange, commonly referred to simply as 'the spice', was a naturally produced awareness spectrum narcotic that formed a fundamental block of commerce and technological development in the known universe for millennia. It also played an important role in travel and cultural development as it is vital for space travel by enabling Navigators to safely guide space ships through fold space.
The spice originated on the planet Arrakis, where it was produced deep beneath the sands. It was created in a process whereby the fungal excretions of sandtrout would mix with water to form a pre-spice mass. This mass would then be brought to the surface of the desert through an explosion of pressure, and under the intense heat and air of Arrakis, melange would form. When the worms died, sandtrout would be released into the sand, and the cycle of creation would repeat.
By the time House Atreides arrived on Arrakis, melange had become the single resource that was capable of creating or destroying an empire. Consequently, Arrakis had gone from being a distant, poor, and unimportant desert world to a greatly prized fief, and management of the spice mining operations were considered a prestigious but difficult task. Spice collection was hindered both by the aggressive and territorial sandworms, and by the guerrilla tactics employed by the Fremen, who resented the presence of off-worlders (especially Imperial agents) who harvested the spice for themselves.