What is "infinitesimal calculus"

Infinitesimal \In`fin*i*tes"i*mal\, a. [Cf. F. infinit['e]simal, fr. infinit['e]sime infinitely small, fr. L. infinitus. See Infinite, a.] Infinitely or indefinitely small; less than any assignable quantity or value; very small.

Infinitesimal calculus, the different and the integral calculus, when developed according to the method used by Leibnitz, who regarded the increments given to variables as infinitesimal.

n. (context calculus English) differential calculus and integral calculus considered together as a single subject.

n. the branch of mathematics that is concerned with limits and with the differentiation and integration of functions [syn: calculus, the calculus]