The practice of priests in being married or having a mistress, i.e. concubinage. It was one of the twin evils to be overthrown in the eyes of the reform movement of 11c Rome, inspired by Pope Gregory VII. (The second evil practice was *simony.) Being sexually active was considered to sully the priest and thus make him unworthy to perform the sacraments. Another anxiety was the prospect of churches and even bishoprics being handed down from father to son. Around 1100 it was not uncommon for rich prebendal churches of St Paul's Cathedral to pass from father to son, e.g. All Saints at St Paul's Walden in Hertfordshire. The term is derived from Nikolaos, a member of the early Christian Church who advocated pagan worship. Revelation 2:15 refers to 'the doctrine of the Nicolaitans, which thing I hate'.
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.