There's an entire field of philosophy called ontology which includes metaontology and it answers your question as "many definitions." I recommend this book [1]

There's an entire field of philosophy called ontology which includes metaontology and it answers your question as "many definitions." I recommend this book [1] by Berto and Plebani. SEP [2] also has about a dozen articles on the ontology of quantum mechanics and quantum physics more generally. [1]. https://www.bloomsbury.com/us/ontology-and-metaontology-9781472573308/ https://www.bloomsbury.com/us/ontology-and-metaontology-9781... [2]. https://plato.stanford.edu/contents.html#q https://plato.stanford.edu/contents.html#q