Definition: The Logos is the second person in the godhead of the Trinity–Jesus. The Logos is the preexistent person of Jesus.
More about the term: Logos is the reason or mind of God (not to be confused with unitarianism). It is the creative force behind the creation of the world, which in turn, gives the world its rational structure. The reason why the world is a logical place open to rational investigation is because it bears the imprint of the Logos (the reason or rationality of the God who created it).
God the Father existed without the universe but having within Him His Word or reason. This proceeded forth from Him (Just as the Holy Spirit proceeds from the mind of God). The pre-incarnate Christ (John 3:13, 31), Son of God, exists as the mind and reason of the Father (eternally rational). The Holy Spirit flows out forth from Him and returns like a ray of the sun. There is only one God. God has certain aspects in His being, or His wisdom, which precedes forth as distinct individuals. The begetting of the Son from the Father is eternal (The sun exists with its sunbeams, but the sunbeams cannot exist without the sun and vise versa [contributed by Origen]).
Example of Use: In the beginning was the Word, and the Word was with God, and the Word was God. (Jn. 1.1)