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.

Source : WordNet®

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