Kief

Kief is a sticky crystal resin that coats the buds of the cannabis plant. After the cannabis is harvested and dried, kief turns into a darker (usually golden to brown) powder. The kief is formed by the glands of the cannabis flower, which secrete oils rich in compounds such as THC, CBD, terpenes and others.
Kief is highly efficient and often "saved" from mills that have a "kief catcher", which is a specific chamber that collects the kief when breaking off the buds. Kief is also a staple ingredient in hash, which is essentially tightly packed pieces of kief.