The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

infinitesimal calculus
    n 1: the branch of mathematics that is concerned with limits and
         with the differentiation and integration of functions [syn:
         calculus, infinitesimal calculus]