Infinitesimal

Part of speech: noun (n.)

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

Part of speech: adjective (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.