A coven was a body of witches which traditionally convened together. More generally, a coven referred to any group of witches or pagans.
As Ted Moss of Fetchborough pointedly outlined to newcomer Maximillian Stael, coven's usually had thirteen members. Stael in fact led this coven, dedicated to the unwilling transformation of Thea Ransome from a human into the Fendahl Core. (TV: Image of the Fendahl)
In the small English village of Moreton Harwood, Bill Pollock and Lily Gregson acted as High Priest and Priestess of the Hecate Cult, a coven dedicated to the goddess Hecate, which Sarah Jane Smith and K9 Mark III disbanded. (TV: A Girl's Best Friend)