Calculus" used to be a more general word meaning "A system or method of calculation, ‘a certain way of performing mathematical investigations and resolutions
differential calculus, integral calculus
Isaac Newton and Gottfried Leibniz.
"the calculus of infinitesimals
The word "calculus" comes from Latin (calculus) and refers to a small stone used for counting
"the science of fluxions".
computations involving velocity and acceleration, the slope of a curve, and optimization