Ambergris is also called Ambra grisea, Ambre gris, ambergrease, or grey amber.
It is a substance regurgitated by sperm whales, possibly to help rid their digestive tract of undigestable materials. When it is first produced, it is quite a bit like poop and rather unpleasant but after spending some time in the sun and sea, it becomes more pleasant, waxy and fragrant.
The smell is earthy and sweet and it was once widely used in perfumery as a fixative. However, due to the declining numbers of wild sperm whales, it has largely been replaced by synthetics.
Historically it has been used to flavor food, as an aphrodisiac, and as medicine for colds, epilepsy, and headaches.
Magically, it can also be used to increase psychic ability and for dream magic.