Neoplatonism is a modern term for a strand of Platonic philosophy that started with Plotinus in the 3rd century CE. Neoplatonic philosophy derives the whole of reality from a single principle, "the One," an idea which is still popular in modern-day spirituality. 

Three distinct phases in Classical Neoplatonism after Plotinus can be distinguished: the work of his student Porphyry; that of Iamblichus and his school in Syria; and the period in the fifth and sixth centuries, when the Academies in Alexandria and Athens flourished. The work of Proclus (412-485) had a lasting influence in the dissemination of Neoplatonism after the closing of the Platonic Academy in Athens in 529 CE by Justinian I. 

In the Middle Ages, Neoplatonic ideas were studied and discussed by Islamic, Christian, and Jewish thinkers. In the Islamic cultural sphere, Neoplatonic texts were available in Arabic translations, and notable thinkers such as al-Farabi, Solomon ibn Gabirol ("Avicebron"), Avicenna and Moses Maimonides incorporated Neoplatonic elements into their own thinking. 

Latin translations of Late Ancient Neoplatonic texts were first available in the Christian West in the 9th century, and became influential from the 12th century onward. Thomas Aquinas had direct access to works by Proclus, Simplicius and Pseudo-Dionysius the Areopagite, and he knew about other Neoplatonists, such as Plotinus and Porphyry, through secondhand sources. Meister Eckhart was also influenced by Neoplatonism, propagating a contemplative way of life which points to the Godhead beyond the nameable God.
Who?
the work of his student Porphyry; that of Iamblichus 