A phenomenon (from Greek φαινόμενoν, pl. φαινόμενα - phenomena) is any observable occurrence. In popular usage, a phenomenon often refers to an extraordinary event. In physics, a phenomenon may be a feature of matter, energy, or spacetime. For example, Isaac Newton made observations of the phenomenon of the moon's orbit. Additionally, Galileo Galilei made observations of pendulum related phenomena.
The term came into its modern philosophical usage through Immanuel Kant, where it is contrasted with noumenon (for which Kant used the term "Ding an sich", or "thing-in-itself") or Absolute. Phenomenon and noumenon serve as interrelated technical terms in Kant's philosophy. Noumena, in contrast to phenomena, are not directly accessible to observation. Nowadays, "phenomena" are often, but not always, understood as 'appearances'. These are themselves sometimes understood as involving qualia.