Lucifer is a Latin word meaning "light-bearer" (from lux, lucis, "light", and ferre, "to bear, bring"), a Roman astrological term for the "Morning Star", the planet Venus. The word Lucifer was the direct translation of the Septuagint Greek heosphoros, ("dawn-bearer"); (cf. Greek phosphoros, "light-bearer") and the Hebrew Helel, ("Bright one") used by Jerome in the Vulgate, having mythologically the same meaning as Prometheus who brought fire to humanity.
That passage, Isaiah 14:12 (see below) referred to one of the popular honorific titles of a Babylonian king; however, later interpretations of the text, and the influence of embellishments in works such as Dante's Inferno and Milton's Paradise Lost, led to the common idea in Christian mythology and folklore that Lucifer was a poetic appellation of Satan.
In modern and late Medieval Christian thought, Lucifer is usually a fallen angel identified as Satan, the embodiment of evil and enemy of God. In Christian literature and legend, Lucifer is generally considered to have been a prominent archangel in heaven (although some sources (Book of Ezekiel 28:14) say he was a cherub or a seraph), who had been motivated by pride to lead a revolution against God, in "The War of Heaven". When the rebellion failed, Lucifer was cast out of heaven, along with a third of the heavenly host, and came to reside in the world.