Aštra’el
Aštara, Blade of Aštoreth
Technically, Lucifer is a translation of another word which also means bearer of light/day star/morning star in Isaiah 14:12.
Usually, when this question is asked, the one asking has a response ready which is meant to show that it is not referring to Satan -or something similar.
The preceding verses make it clear that Isaiah 14:12 is addressing the King of Babylon. Christians have often interpreted it as an indirect reference to the devil's alleged fall from grace (another Christian interpretation), but scripture makes it clear that the King of Babylon is the subject in question.