A Greek term denoting "mind" or "reason" used by Greek philosophers beginning with Anaxagoras, and later by Plato and Neo-Platonism generally.

In History in English Words, Barfield provides a capsule history of the concept:

The Stoics were the first to identify this human faculty with that divine Mind (Nous) which earlier Greek philosophy had perceived as pervading the visible universe. They were the first to make the progressive incarnation of thought in audible sound a part of the creative working of God in the world; and it is to them accordingly, with their deep sense of the divine significance of words and their origin, that we owe the word etymology, the first half of which is composed of a poetical Greek adjective meaning "true." Though he had never heard of Christianity, Philo, importing into the theory a certain Semitic awfulness, actually called this mysterious "logos" the "only begotten-son." (113)

See in particular "Thinking and Thought" (RCA 47-66), "Philosophy and Religion" (HEW 96-117).