Yantra
Yantra and mandala are often treated as interchangeable in Western usage, but in tantric usage a mandala may serve any deity while a yantra belongs to one particular deity. European study of yantras as working ritual instruments was already under way in John Woodroffe's Tantra publications of the 1910s, before Heinrich Zimmer's 1926 study.
Arthur Avalon (John Woodroffe), ed., Principles of Tantra (1914–16), on the mandala and the yantra; John Woodroffe, Shakti and Shakta (1918); Heinrich Zimmer, Kunstform und Yoga im indischen Kultbild (1926)
A broad category of geometric ritual diagrams used in Hindu tantric practice, of which the famous Sri Yantra is just one specific example.