calculus (ˈkælkjʊləs)  
—n , pl luses, li  
1.  a branch of mathematics, developed independently by Newton and Leibniz. Both differential calculus and integral calculus are concerned with the effect on a function of an infinitesimal change in the independent variable as it tends to zero 
2.  any mathematical system of calculation involving the use of symbols 
3.  logic Compare formal language an uninterpreted formal system 
4.  pathol a stonelike concretion of minerals and salts found in ducts or hollow organs of the body 
[C17: from Latin: pebble, stone used in reckoning, from calx small stone, counter] 
calculus [%PREMIUM_LINK%] (kāl'kyələs) Pronunciation Key Plural calculi (kāl'kyəlī') or calculuses

The branch of mathematics, usually studied after algebra, that provides a natural method for describing gradual change.
Note: Most modern sciences use calculus.