The word logo turned up in English in 1937, probably a shortening of logogram, meaning a "sign or character representing a word" (1820), derived from Greek logos "word" + gram "what is written." A logogram is a single written character (a glyph) which represents a complete grammatical word or morpheme.
The term logo has several meanings:
Used synonymously with the word logotype (q.v.), it is a visual device that graphically defines a (usually corporate) identity, incorporating an emblem or symbol, a name, acronym or part of a name often in a unique font, or both, protected by copyright. See under logotype. See also trademark.
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Heraclitus also used Logos to mean the undifferentiated material substrate from which all things came: "Listening not to me but to the Logos it is wise to agree that all [things] are one." In this sense Logos is Heraclitus' answer to the Pre-Socratic question of what the arche is of all things.
The Stoics understood Logos as the animating power of the universe, (as it is also presently understood today in Theosophical terms and by the Rosicrucians in their conception of the cosmos) which further influenced how this word was understood later on (in 20th century psychology, for instance).
In rhetoric, logos is one of the three modes of persuasion (the other two are pathos, emotional appeal; and ethos, the qualification of the speaker or distinguished character sentiment or morality).