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 (c.  535 – c.  475 BC), who used the term for a principle of order and knowledge. Logos is the logic behind an argument. Logos tries to persuade an audience using logical arguments and supportive evidence. Logos is a persuasive technique often used in writing and rhetoric.

Ancient 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. Stoic philosophers identified the term with the divine animating principle pervading the Universe. Within Hellenistic Judaism, Philo of Alexandria (c. 20 BC – c 50 AD) adopted the term into Jewish philosophy. The Gospel of John identifies the Logos, through which all things are made, as divine (theos), and further identifies Jesus Christ as the incarnate Logos. The term is also used in Sufism, and the analytical psychology of Carl Jung.

Despite the conventional translation as "word", it is not used for a word in the grammatical sense; instead, the term lexis (léxis) was used. However, both logos and lexis derive from the same verb légō (λέγω), meaning "(I) count, tell, say, speak".

Author and Professor Jeanne Fahnestock describes logos as a "premise". She states that, to find the reason behind a rhetor's backing of a certain position or stance, one must acknowledge the different "premises" that the rhetor applies via his or her chosen diction. The rhetor's success, she argues, will come down to "certain objects of agreement...between arguer and audience". "Logos is logical appeal, and the term logic is derived from it. It is normally used to describe facts and figures that support the speaker's topic." Furthermore, logos is credited with appealing to the audience's sense of logic, with the definition of "logic" being concerned with the thing as it is known. Furthermore, one can appeal to this sense of logic in two ways. The first is through inductive reasoning, providing the audience with relevant examples and using them to point back to the overall statement. The second is through deductive enthymeme, providing the audience with general scenarios and then indicating commonalities among them.

Philo distinguished between logos prophorikos ("the uttered word") and the logos endiathetos ("the word remaining within"). The Stoics also spoke of the logos spermatikos (the generative principle of the Universe) which foreshadows related concepts in Neoplatonism. 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 (perhaps inadequate) 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.