Dried and powdered sour green mango. Sometimes known as amchur, this souring agent and spice is known only in Indian cuisine.
In Hindi "amm" is mango, "choor" is powder.
Mangoes are native to India, and when their season comes around each year, there will be a glut of wild mangoes. It is generally these mangoes, picked very unripe when their sugar levels has not had a chance to balance the acidity, that are peeled, sliced, sun dried and powdered. It is sometimes possible to buy the dried slices to powder yourself or add whole to soups and curries. These slices look like small pieces of dry, dead wood.
Amchoor has two main uses. As an acidulant it has a mouth puckering sourness, obviously without the freshness of lemon juice, but with an added depth of flavour. It is a very common ingredient in Indian pickles. Secondly, It is included in marinades where some sour flavour is imparted, but also acting as a tenderizer.
Depending on the strength of flavour required, amchoor powder will either be fried in oil at the start of a dish, providing a richer flavour, or added towards the end of cooking, resulting in a more subtle taste.