Lucifer is a Latin adjective meaning “light-bringer”. It was used of both the moon and more frequently of the planet Venus, the “morning star”, the brightest object in the sky just before dawn.
Early Christian usage
In Christian usage the word became used as a name widely from the Latin of 2 Peter 1:19 “lucifer oriatur in cordibus vestris” (the morning-star arise in your hearts). This was taken as a title of Christ and features in early hymns such as that by the 4th Century bishop Hilary of Poitiers with the line “Tu verus mundi lucifer” (you are the true light bringer of the world). At the same time Lucifer became a popular Christian name, including of two early bishops.
Later Christian usage
A sudden change in the associations of name came about in the 5th Century with increasing reference to another “morning star” (Latin lucifer) in the Latin Bible, namely the king of Babylon in Isaiah 14:12
Quomodo cecidisti de caelo, lucifer, fili aurorae
This literally is “How you are fallen from heaven, O Day Star, son of Dawn!” (NRSV) and from the context has nothing to do with angels – Isaiah 14:16-17 calls him a “man”
However it came to be used as a verse about the devil, and the original usage from 2 Peter 1:19 was largely forgotten.
In English Bibles the word was traditionally left untranslated as a name of the devil:
A! Lucifer, that risedest early, how fellest thou down from heaven; (Wycliffe Bible, c. 1390)
How art thou fallen from heaven, O Lucifer, son of the morning… (King James Version, 1611)
Now the clock has turned back again to the early Christian understanding of these verses, and modern versions translate the word as “morning star” in both Isaiah 14 and 2 Peter 1.
The comparison to Venus is quite suitable; the king of Babylon was very proud and imagined himself to be a god. He said “I will make myself like the Most High” (Isaiah 14:14) and apparently thought of himself as being “in heaven” like Venus. Instead, he fell to the earth in defeat. Isaiah describes him as a metaphorical morning star — appearing bright and elevated, but about to plunge below the horizon and disappear.
A similar passage is Ezekiel 28 which is about the king of Tyre, although it is also often incorrectly interpreted as applying to a wicked angel.