Is Satan an angel or a Jinn?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

The correct opinion of scholars is that Satan was not one of the angels - he was a Jinn. Allaah the Almighty Says (what means): {And [mention] when We said to the angels, "Prostrate to 'Aadam (Adam)," and they prostrated, except for Iblees (Satan). He was of the Jinn and departed from the command of his Lord.} [Quran 18:50]

Al-Hasan Al-Basri stated that Satan was never one of the angels and that he is in fact the father of the Jinn, just like 'Aadam is the father of humankind. [Ibn Jareer [Saheeh (Authentic)]

Allaah Knows best.