For the purposes of this knowledgebase, entity is the origin item. It starts the chain of subclasses that are used to organize a knowledge representation of everything this knowledgebase contains and describes. It is analogous to "thing" in the schema.org specifications. It is also generally analogous to the Wikidata concept of entity. Other classification schemes adopted and adapted into this knowledgebase trace their root to this item.
Developing this knowledgebase is an evolutionary process where we are attempting to organize a representation of everything we know about the earth system, not in a theoretical or philosophical sense, but as a living data, information, and knowledge instrument we are working to apply in the work of the research science conducted by the USGS. As such the organization of items and properties used to represent what we know as they relate to this "top level" item is fluid through time. It continues to improve as we test methods of organizing what we know and leverage the many formal ontologies that are being developed in our broad set of scientific domains.