What is the term used for the study of the nature of existence?

The term that refers to the study of the nature of existence is ontology. This area of philosophy focuses on questions regarding what entities exist and how they can be categorized and related within a hierarchy. It delves into concepts such as being, existence, and reality, exploring the fundamental nature of things.

Ontology is foundational in various fields, including philosophy, metaphysics, and even in research methodology, as it influences how researchers conceive of the constructs they study. It sets the stage for other philosophical inquiries, including how we come to understand and know those entities, which is where epistemology comes into play.

Epistemology revolves around the nature of knowledge itself—how we know what we know and the limits of our understanding—but it does not address the existence of things directly. Constructivism pertains to the theory of knowledge and the understanding that individuals construct their own understanding and knowledge through experiences. Critical theory critiques society and culture by applying knowledge from the social sciences, and while it may touch on aspects of existence in a societal context, it does not specifically study the nature of existence.

Thus, ontology is distinctly focused on the foundational questions of existence and reality, making it the correct term in this context.