Traditionally, Lucifer is a name that in English generally refers to the devil before being cast from heaven, although this is not the original meaning of the term.
In Latin, from which the English word is derived, Lucifermeans "light-bearer".
It was the name given to the dawn appearance of the planet Venus, which heralds daylight.
For this meaning, English generally uses the names "Morning Star" or "Day Star", and rarely "Lucifer".
The Bible does not name the devil as Lucifer.
The use of this name in reference to the devil stems from an interpretation of Isaiah 14:3-20, a passage that does not speak of any fallen angel but of the defeat of a particular Babylonian King, to whom it gives a title that refers to what in English is called the Day Star or Morning Star.
Isaiah 14:3-20
3When the LORD has given you rest from your pain and turmoil and the hard service with which you were made to serve,
4you will take up this taunt against the king of Babylon: "How the oppressor has ceased, the insolent fury ceased!
5The LORD has broken the staff of the wicked, the scepter of rulers,
6that struck the peoples in wrath with unceasing blows, that ruled the nations in anger with unrelenting persecution.
7The whole earth is at rest and quiet; they break forth into singing.
8The cypresses rejoice at you, the cedars of Lebanon, saying, 'Since you were laid low, no woodcutter comes up against us.'
9Sheol beneath is stirred up to meet you when you come; it rouses the shades to greet you, all who were leaders of the earth; it raises from their thrones all who were kings of the nations.
10All of them will answer and say to you: 'You too have become as weak as we! You have become like us!'
11Your pomp is brought down to Sheol, the sound of your harps; maggots are laid as a bed beneath you, and worms are your covers.
12"How you are fallen from heaven, O Day Star, son of Dawn! How you are cut down to the ground, you who laid the nations low!
13You said in your heart, 'I will ascend to heaven; above the stars of God I will set my throne on high; I will sit on the mount of assembly in the far reaches of the north;
14I will ascend above the heights of the clouds; I will make myself like the Most High.'
15But you are brought down to Sheol, to the far reaches of the pit.
16Those who see you will stare at you and ponder over you: 'Is this the man who made the earth tremble, who shook kingdoms,
17who made the world like a desert and overthrew its cities, who did not let his prisoners go home?'
18All the kings of the nations lie in glory, each in his own tomb;
19but you are cast out, away from your grave, like a loathed branch, clothed with the slain, those pierced by the sword, who go down to the stones of the pit, like a dead body trampled underfoot.
20You will not be joined with them in burial, because you have destroyed your land, you have slain your people.
In 2 Peter 1:19 and elsewhere, the same Latin word lucifer is used to refer to the Morning Star, with no relation to the devil.
2 Peter 1:19
19 We also have the prophetic message as something completely reliable, and you will do well to pay attention to it, as to a light shining in a dark place, until the day dawns and the morning star rises in your hearts.
It is only in post-New Testament timesthat the Latin word Lucifer was used as a name for the devil, both in religious writing and in fiction, especially when referring to him prior to his fall from Heaven.
