Source : Webster's Revised Unabridged Dictionary (1913)

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.

Infinitesimal \In`fin*i*tes"i*mal\, n. (Math.)
   An infinitely small quantity; that which is less than any
   assignable quantity.

Source : WordNet®

infinitesimal
     adj : infinitely or immeasurably small; "two minute whiplike
           threads of protoplasm"; "reduced to a microscopic
           scale" [syn: {minute}, {microscopic}]