Nicolaitans are a group mentioned in the Book of Revelation when Jesus rebuked the churches of Ephesus and Pergamum.
Rev 2:6Yet this you have: you hate the works of the Nicolaitans, which I also hate.
Rev 2:15So you also have those who hold the doctrine of the Nicolaitans, which thing I hate.
Apostle John stated that they caused believers to eat food sacrificed to idols and to commit sexual immorality, but there is no other clear explanation.
Rev 2:14But I have a few things against you, because you have there those who hold the doctrine of Balaam, who taught Balak to put a stumbling block before the children of Israel, to eat things sacrificed to idols, and to commit sexual immorality.Rev 2:15Thus you also have those who hold the doctrine of the Nicolaitans, which thing I hate.
Traditionally, there is a theory that Nicolas, one of the seven deacons of the early church, and his followers fell into heresy, but this is also not certain.