Webster's Revised Unabridged Dictionary · 1913

Infinitesimal

Infinitesimal , adjective

[Compare French infinitésimal, from infinitésime infinitely small, from Latin infinitus. See Infinite, a.]

Infinitely or indefinitely small; less than any assignable quantity or value; very small.
Collocations (1)
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.

Infinitesimal , noun

(Mathematics) An infinitely small quantity; that which is less than any assignable quantity.