Borromean nuclei are those where three separate parts of the nucleus are mutually bound together, but where, if any one were removed, the remaining two would be un-bound (without any binding energy). The term is derived from a Borromean Ring, which consists of three mutually interlocking rings which will fall apart if any one is removed, but it is not implied that this is the physical construction of Borromean nuclei. By way of information, three rings can also be mutually interlocked in such a way that removing one leaves the other two still linked, but by definition, these are not Borromean Rings.

Carbon-12, in its excited isomeric state, is Borromean, because the three sub-units of helium-4 within its nucleus are bound together, whereas the two helium-4 sub-units in beryllium-8 are not bound. Strictly speaking, beryllium-8 isn't a nuclide at all because it is un-bound (possesses a positive not negative binding energy).

Those Borromean nuclei with neutron halos have two separate extra neutrons, but those with only one extra neutron cannot be Borromean, by definition. The neutron halo nucleus helium-6 with its helium-4 core and two separate extra neutrons is Borromean, as are the other neutron halo nuclides lithium-11, beryllium-14, and carbon-22. They are Borromean because their lighter cousins with just one extra neutron (lithium-10, beryllium-13, and carbon-21) are un-bound. Halo nuclei with just one halo neutron (like beryllium-11 and carbon-19) cannot be Borromean.

See Neutron Halo nuclei.