Yantra (यन्त्र) is the Sanskrit word for a mystical diagram, especially diagrams from the Tantric traditions of the Indian religions. They are used for worship of deities in temples or at home; as an aid in meditation; used for the benefits given by their supposed occult powers based on Hindu astrology and tantric texts.
In Classical Sanskrit, the generic meaning of yantra is “instrument, contrivance, apparatus”. In Rigvedic Sanskrit, it meant an instrument for restaining or fastening, a prop, support or barrier, etymologically from the root yam “to sustain, support” and the -tra suffix expressing instruments. The literal meaning is still evident in the medical terminology of Sushruta, where the term refers to blunt surgical instruments such as tweezers or a vice. The meaning of “mystical or occult diagram” arises in the medieval period (Kathasaritsagara, Pancharatra).