What does "indivisible" mean?

Indivisible: (Math.) Not capable of exact division, as one quantity by another; incommensurable.

Additional senses

  1. 2.That which is indivisible. By atom, nobody will imagine we intend to express a perfect indivisible, but only the least sort of natural bodies. Digby.
  2. 3.(Geom.) An infinitely small quantity which is assumed to admit of no further division. 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.

Sources