The Collaborative International Dictionary of English v.0.48:

Indivisible \In`di*vis"i*ble\, n.
   1. That which is indivisible.
      [1913 Webster]

            By atom, nobody will imagine we intend to express a
            perfect indivisible, but only the least sort of
            natural bodies.                       --Digby.
      [1913 Webster]

   2. (Geom.) An infinitely small quantity which is assumed to
      admit of no further division.
      [1913 Webster]

   Method of indivisibles, a kind of calculus, formerly in
      use, in which lines were considered as made up of an
      infinite number of points; surfaces, as made up of an
      infinite number of lines; and volumes, as made up of an
      infinite number of surfaces.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Indivisible \In`di*vis"i*ble\, a. [L. indivisibilis: cf. F.
   indivisible. See In- not, and Divisible.]
   [1913 Webster]
   1. Not divisible; incapable of being divided, separated, or
      broken; not separable into parts. "One indivisible point
      of time." --Dryden.
      [1913 Webster]

   2. (Math.) Not capable of exact division, as one quantity by
      another; incommensurable.
      [1913 Webster]

WordNet (r) 3.0 (2006):

indivisible
    adj 1: impossible of undergoing division; "an indivisible union
           of states"; "one nation indivisible" [ant: divisible]

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "indivisible":
   a certain, an, any, any one, atomic, austere, bare, basic,
   bound up in, bound up with, chaste, coherent, cohesive, either,
   elementary, essential, exclusive, fundamental, homely, homespun,
   homogeneous, impartible, inalienable, indiscerptible, indissoluble,
   indissolvable, individual, infrangible, infusible, inseparable,
   insoluble, integral, irreducible, lone, mere, monadic, monistic,
   monolithic, of a piece, one, plain, primal, primary, pure,
   pure and simple, severe, simon-pure, simple, single, singular,
   sole, solid, solitary, spare, stark, unadorned, unanalyzable,
   uncluttered, undifferenced, undifferentiated, undividable,
   undivided, unified, uniform, unique, unitary, whole