"The Yantra Mandir (commonly known as the Jantar Mantar) is an equinoctial dial, consisting a gigantic triangular gnomon with the hypotenuse parallel to the Earth's axis. On either side of the gnomon is a quadrant of a circle, parallel to the plane of the equator. The instrument is intended to measure the time of day, correct to half a second,citation needed and declination of the Sun and the other heavenly bodies."
– from the Wikipedia article (17 Jan 2007)
There were five such "dials" built by the Maharaja Jai Singh II of Jaipur in the 1720s and 1730s. The most famous ones are in Jaipur and Delhi. The word means "calculation instrument" (also from Wikipedia).