California Code, Fish and Game Code - FGC § 2721

(a) A unique species or natural community, whose existence at a single location in California is the only known occurrence in the world of that particular species or natural community.

(b) A species that occurs in only 20 or fewer locations in the world, at least one of which is in California.

(c) A natural community that occurs in only 50 or fewer locations in the world, at least one of which is in California.

(d) An assemblage of three or more highly rare species or natural communities, or any combination thereof, of which at least one of the species or natural communities is found only in 20 or fewer locations in the world.