A system of theological and mystical philosophy founded in the third century by Plotinus, and developed by a number of his contemporaries or near contemporaries, including Iamblichus, Porphyry, and Proclus. It is also influenced by a variety of other systems of thought, including Stoicism and Pythagoreanism.
The teachings are heavily based upon the works of Plato, a philosopher in Classic Greece. In Plotinus's time, they would have simply been termed "Platonists." Modern understandings have led us to create a new word to separate this system of thought from the one taught by Plato.
While the early Neoplatonists were pagans, many Neoplatonists ideas influenced both mainstream Christian and Gnostic beliefs.
Neoplatonist beliefs are centered on the idea of a single supreme source of goodness and being in the universe from which all other things descend. Every iteration of an idea or form becomes less whole and less perfect. Neoplatonists also accept that evil is simply the absence of goodness and perfection. Finally, Neoplatonists support the idea of a world soul, which bridges the divide between the realms of forms and the realms of tangible existence.
Such writers as Marslio Ficino, Giordano Bruno, and Giovanni Pico della Mirandola revived Neoplatonism during the Renaissance.