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]