A mortar and pestle is a tool used to grind and mix substances. The pestle is a heavy stick whose end is used for pounding and grinding, and the mortar is a bowl. The substance is ground between the pestle and the mortar.
Without each other, they are useless. How beautiful.
The English "pestle" derives from classical Latin pistillum, meaning pounder. The classical Latin mortarium led to the English mortar, meaning, among several other usages, "receptacle for pounding" and "product of grinding or pounding".
The Roman poet Juvenal applied both "mortarium" and "pistillum" to articles used in the preparation of drugs, reflecting the early use of the mortar & pestle as a pharmacist's or apothecary's symbol.
Today the act of mixing ingredients or reducing the particle size is known as trituration. Mortars and pestles are also used as drug paraphernalia by some in order to grind up pills to speed up absorption when they are ingested or in preparation for snorting.