Yantra is a Sanskrit word that is derived from the root yam meaning to control or subdue or "to restrain, curb, check". Meanings for the noun derived from this root include:
"any instrument or machine" (i.e. that which is controlled or controls. For instance the body is said to be a yantra)
"any instrument for holding, restraining, or fastening" (for instance a symbol which 'holds' the essence of a concept, or helps the mind to 'fasten' on a particular idea)
"a mystical or astronomical diagram" (usually a symbol, often inscribed on an amulet) sometimes said to possess mystical or magical powers.
-tra is an indoeuropean suffix meaning ' instrument', found in Latin aratrum and in tantra and mantra. A yantra depicts both macrocosmic and microcosmic forces acting together - the movement towards and away from the centre - "control" and "liberation" within the one device. Mantra plus yantra creates tantra. In some disciplines of Tantra it is said that a focused, controlled gaze upon a particular yantra may lead to liberation.