The chemical bond is among the oldest ideas in chemistry and still among the least settled: chemists continue to disagree about what a bond is. We argue that the recurring controversies (ionic versus covalent, dative versus shared-electron, hypervalent versus metavalent, whether metal–metal contacts are genuinely collective) are not disputes about the physics but about an unstated reference against which a bond is declared. Removing the reference forces a change of viewpoint: an atom in a molecule is not a fixed piece of electron density but an open quantum system, a mixed state that is a probabilistic superposition over how many electrons it owns. From this vantage the electronnumber fluctuations that the theory of electron distribution functions has long measured acquire a sharp meaning: they are the quantum correlation, the entanglement, shared between the atoms of a molecule. Measured directly between the Bader basins chemists already draw, this entanglement yields a single, refere