1. 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. 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.