Calculus \KAL-kyuh-luhs\ , noun;
1. Mathematics: A method of calculation, especially one of several highly systematic methods of treating problems by a special system of algebraic notations, as differential or integral calculus
2. Pathology: A stone, or concretion, formed in the gallbladder, kidneys, or other parts of the body
3. Dentistry: A hard, yellowish to brownish-black deposit on teeth formed largely through the mineralization of dead bacteria in dental plaques by the calcium salts in salivary secretions and subgingival transudates - also called tarter
4. Calculation; estimation or computation
This word dates to the 1660's from Latin calculus with the meaning "reckoning, account," but it originally meant "pebble used as a reckoning counter." The earlier definition makes sense when you know that calculus is a diminutive of calx, which is "limestone." Calx derives from Greek khalix ("small pebble"), which has been traced to a Proto-Indo-European root for "split, break up."
The dental and pathology meanings trace their meaning to a 1732 usage meaning "concretion occuring accidentally in the animal body." The mathematics sense is actually a shortening of differential calculus, and classes of this name have been the bane of many students' existences since the 18th century.