A phenomenon (from Greek φαινόμενoν), plural phenomena, is any observable occurrence. Phenomena are often, but not always, understood as “appearances” or “experiences“. These are themselves sometimes understood as involving qualia.
The term came into its modern philosophical usage through Immanuel Kant, who contrasted it with noumenon (for which he used the term “Ding an sich“, or “thing-in-itself“), which, in contrast to phenomena, are not directly accessible to observation. Kant was heavily influenced by Leibniz in this part of his philosophy, in which phenomenon and noumenon serve as interrelated technical terms.