1. an observable event or physical occurrence.

2. in philosophy, something perceived by the senses. In Greek philosophy, most notably that of Plato (c. 427–c. 347 bce), phenomena are the sensible things that constitute the world of experience, as contrasted with the transcendent realities that are known only through reason. Immanuel Kant used the term phenomena to refer to things as they appear to the senses and are interpreted by the categories of human understanding. For Kant, knowledge of phenomena is the kind of knowledge available to human beings; in contrast, knowledge of noumena, or things in themselves, remains beyond human experience or reason.

3. an occurrence or entity that defies explanation. —phenomenal adj.