1660s, from Latin calculus "reckoning, account," originally "pebble used as a reckoning counter," diminutive of calx (genitive calcis) "limestone" (see chalk, n.). Modern mathematical sense is a shortening of differential calculus. Also used from 1732 to mean kidney stones, etc., then generally for "concretion occurring accidentally in the animal body," such as dental plaque. Related: Calculous (adj.).