calculus (noun) – calculation or computation; specific branches of mathematics such as differential calculus and integral calculus
BREAKDOWN: CALC- (pebble used for counting) + -ULUS (small)
— a calculus is also a stony concretion that forms in a body or on teeth (plural: calculi)
This word is built on the root CALC-, which means small stone, pebble used for counting, lime or calcium, or heel.
Common CALC- words include calcium, calculator, chalk, and miscalculate.
More esoteric CALC- words include calx, calcareous, and calculous.