A yellowish secretion from the castor sacs (a type of scent gland) of North American beaver and European beaver.
Etymology: from Greek kastor, beaver.
It is mainly used in some perfumes, and in trapping (to lure beavers to the trap).
Although allowed, it is rarely - if at all - used as a flavoring in food (per snopes).
Wikipedia does mention its use in cigarettes and in a type of schnapps.
Not to be confused with castor oil, which is pressed from the seeds of the castor oil plant (Ricinus communis).
(The origin of the name "castor oil" seems unclear.)