Costus is the root of Saussurea costus, a tall thistle-like plant of the western Himalaya, and it is one of the very few aromatics that appears as a standard drug in Greek, Roman, Indian, Chinese and Arabic medicine at the same time. Dioscorides has it, the Ayurvedic texts have it as kuth or kushtha, Chinese medicine has it as mu xiang, and the Arabic materia medica has it as qust.
Its position in Islamic tradition is unusual for a plant. Several hadith recommend qust al-hindi, Indian costus, and describe it as among the best remedies available, which gives it a standing within prophetic medicine that ordinary aromatics do not have. It is still widely sold across the Muslim world on that basis, powdered, and used as a snuff, an oil and a burned incense.
In Indian practice it is an ingredient of compounded temple incense and a drug for skin conditions, which the Sanskrit name may reflect.
The plant is CITES Appendix I listed, which is the strictest category, following collapse of the wild Kashmiri populations. Cultivation in Himachal Pradesh and elsewhere now supplies the legal trade, and a good deal of what circulates in the herbal trade is another Saussurea species, or is not Saussurea at all.