Stewart Robert Hinsley
Hybrids formed by somatic cell fusions such as you describe are also
possible [especially in laboratory cell cultures], but they're
a separate thing from "graft hybrids".

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cytohybrids?
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I was also of the opinion that graft hybrids and chimeras were the same,
but I was recently browsing a book which had them as different. It
appears that the original hypothesis for various plants
(Hawthorn/Medlar, Laburnum/Broom) now recognised as chimeras was that they
were true hybrids arising from fusion of cells in the graft, and
therefore described as "graft hybrids". It would appear that there are
two usages for the term "graft hybrid". (And checking B. Daydon
Jackson's "A Glossary of Botanic Terms" I find a 3rd - "effect produced
by one or other of the united individuals on its grafted fellow" - which
would seem to make a fruit tree on a dwarfing rootstock a "graft
hybrid".)
--
Stewart Robert Hinsley

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I would suppose that this is why the ICNCP uses "graft-chimaera" as the
official term?
PvR