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.