Logos is a term in Western philosophy, psychology, rhetoric, and religion derived from a Greek word variously meaning "ground", "plea", "opinion", "expectation", "word", "speech", "account", "reason", "proportion", and "discourse". It became a technical term in Western philosophy beginning with Heraclitus, who used the term for a principle of order and knowledge. Other Greek philosophers used the term in different ways. The sophists used the term to mean discourse. Aristotle applied the term to refer to "reasoned discourse" or "the argument" in the field of rhetoric, and considered it one of the three modes of persuasion alongside ethos and pathos. Pyrrhonist philosophers used the term to refer to dogmatic accounts of non-evident matters. The Stoics spoke of the logos spermatikos (the generative principle of the Universe) which foreshadows related concepts in Neoplatonism.

Within Hellenistic Judaism, Philo (c.  20 BC – c.  50 AD) adopted the term into Jewish philosophy. Philo distinguished between logos prophorikos ("the uttered word") and the logos endiathetos ("the word remaining within").

The Gospel of John identifies the Christian Logos, through which all things are made, as divine (theos), and further identifies Jesus Christ as the incarnate Logos. Early translators of the Greek New Testament such as Jerome (in the 4th century AD) were frustrated by the inadequacy of any single Latin word to convey the meaning of the word logos as used to describe Jesus Christ in the Gospel of John. The Vulgate Bible usage of in principio erat verbum was thus constrained to use the  noun verbum for "word", but later Romance language translations had the advantage of nouns such as le mot in French. Reformation translators took another approach. Martin Luther rejected Zeitwort (verb) in favor of Wort (word), for instance, although later commentators repeatedly turned to a more dynamic use involving the living word as felt by Jerome and Augustine.