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]