Arther Koestler coined the word. It means "whole" in Greek. He then built a theory around this concept.

He observed that nature is architected with a series of parts that were both whole unto themselves and part of a larger whole.

For instance, a hydrogen atom is whole unto itself. One electron flying through a field around a nucleus. But that atom then can become with another hydrogen atom and and oxygen atom, a water molecule. And that water then becomes part of a cellular structure, and that cellular structure then is a part of more advanced forms of life, each being a whole unto themselves.

He calls a holonic structure a "holarchy".

One that can reach from subatomic particles to multiverses.

>Since a holon is embedded in larger wholes, it is influenced by and influences these larger wholes. And since a holon also contains subsystems, or parts, it is similarly influenced by and influences these parts. Information flows bidirectionally between smaller and larger systems as well as through rhizomatic contagion. > When this bidirectionality of information flow and understanding of role is compromised, for whatever reason, the system begins to break down: wholes no longer recognize their dependence on their subsidiary parts, and parts no longer recognize the organizing authority of the wholes. Cancer may be understood as such a breakdown in the biological realm. > A hierarchy of holons is called a holarchy. The holarchic model can be seen as an attempt to modify and modernise perceptions of natural hierarchy. > wikipedia