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]

The Collaborative International Dictionary of English v.0.48:

Infinitesimal \In`fin*i*tes"i*mal\, n. (Math.)
   An infinitely small quantity; that which is less than any
   assignable quantity.
   [1913 Webster]

WordNet (r) 3.0 (2006):

infinitesimal
    adj 1: infinitely or immeasurably small; "two minute whiplike
           threads of protoplasm"; "reduced to a microscopic scale"
           [syn: infinitesimal, minute]
    n 1: (mathematics) a variable that has zero as its limit

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "infinitesimal":
   atomic, corpuscular, embryonic, evanescent, germinal, granular,
   impalpable, imperceptible, imponderable, inappreciable,
   indiscernible, intangible, invisible, microcosmic, microscopic,
   molecular, subatomic, tenuous, thin, ultramicroscopic, unseeable