Struggled with this one for decades. Finally landed upon a perfect set of words.
With “Theory theory” as a scientific basis:
“A concept is a theory of a category.”
Going further, utilizing a computer science usage of Ontology:
“Ontologies are built up of concepts.”
Not all ontologies map cleanly to one another even when the same words or symbols are used, which leads to misunderstandings. (communications).
“Category” may seem to be an undefined term but when viewed from Theory theory, a “theory of a category” defines category.