indivisible

1. not comparableIncapable of being divided; atomic.

   one indivisible point of time

   1695, C[harles] A[lphonse] du Fresnoy, translated by John Dryden, De Arte Graphica. The Art of Painting, […], London: […] J[ohn] Heptinstall for W. Rogers, […], →OCLC

   1 more example

   Although the mainland and Taiwan are yet to be reunited, they belong to one and same China, which is an indivisible whole.

   2014 [2013 June 13], Xi Jinping, “Handle Cross-Straits Relations in the Overall Interests of the Chinese Nation”, in The Governance of China, volume I, Beijing: Foreign Languages Press, →ISBN, →OCLC, page https://archive.org/details/xijinpingthegovernanceofchinavolume1/page/n260

2. not comparablearithmeticIncapable of being divided by a specific integer without leaving a remainder.

plural indivisibles

1. That which cannot be divided or split.

   By atom, nobody will imagine we intend to express a perfect indivisible, but only the least sort of natural bodies.

   1644, Kenelme [i.e., Kenelm] Digby, Two Treatises. In the One of which, the Nature of Bodies; in the Other, the Nature of Mans Soule; is Looked into: In Way of Discovery, of the Immortality of Reasonable Soules, Paris: […] Gilles Blaizot, →OCLC

   1 more example

   The compoſition of Bodies, whether it be of Diviſibles or Indiviſibles, is a queſtion which must be rank'd with the Indiſſolvibles: For though it hath been attempted by the moſt illuſtrious Wits of all Philoſophick Ages; yet they have done little elſe, but ſhewn their own diviſions to be almoſt as infinite, as ſome ſuppoſe thoſe of their Subject.

   1661, Joseph Glanvill, chapter V, in The Vanity of Dogmatizing: Or Confidence in Opinions. […], London: […] E. C[otes] for Henry Eversden […], →OCLC, page 53

2. geometryAn infinitely small quantity which is assumed to admit of no further division.