What is the occult?

Literally it means "knowledge of the hidden" or "knowledge of the paranormal". It basically is anything supernatural that takes place without God, which would imply Satan is the source of it.

Here is what Scripture says about it.

Deut 18:10-12 — ~"There shall not be found among you anyone who makes his son or his daughter pass through the fire, one who uses divination, one who practices witchcraft, or one who interprets omens, or a sorcerer,
or one who casts a spell, or a medium, or a spiritist, or one who calls up the dead.
~"For whoever does these things is detestable to the LORD; and because of these detestable things the LORD your God will drive them out before you.

Lev 19:31 — `Do not turn to mediums or spiritists; do not seek them out to be defiled by them. I am the LORD your God.

Lev 20:6 — `As for the person who turns to mediums and to spiritists, to play the harlot after them, I will also set My face against that person and will cut him off from among his people.

Lev 20:27 — `Now a man or a woman who is a medium or a spiritist shall surely be put to death. They shall be stoned with stones, their bloodguiltiness is upon them."'

2 Chronicles 33:6 — He made his sons pass through the fire in the valley of Ben-hinnom; and he practiced witchcraft, used divination, practiced sorcery and dealt with mediums and spiritists. He did much evil in the sight of the LORD, provoking Him {to anger.}

2 Kings 21:6 — He made his son pass through the fire, practiced witchcraft and used divination, and dealt with mediums and spiritists. He did much evil in the sight of the LORD provoking {Him to anger.}

Galatians 5:19-21 — Now the deeds of the flesh are evident, which are: immorality, impurity, sensuality,
idolatry, sorcery, enmities, strife, jealousy, outbursts of anger, disputes, dissensions, factions,
envying, drunkenness, carousing, and things like these, of which I forewarn you, just as I have forewarned you, that those who practice such things will not inherit the kingdom of God.

Matthew 4:10 — Then Jesus *said to him, "Go, Satan! For it is written, `YOU SHALL WORSHIP THE LORD YOUR GOD, AND SERVE HIM ONLY."'