A group of witches is known as a coven, and it was suggested throughout the biggest part of the 20th Century that witches in Europe met in covens consisting of 13 witches. In modern times, followers and practitioners of Wicca, who are known as Wiccans, gather in covens in much the same way that Christians gather in congregations.
According to the Oxford English Dictionary, it is thought that the word "coven" originated in the medieval Scotland around the year 1500, and it was originally used to denote a gathering of any kind. It is a derivative of the Latin "to come together or to gather," which is "convenire." This Latin etymology is also behind the word "convene."
The word "coven" was first recorded in use during the trial of a Scottish witch in 1662 — Isobel Gowdie. The term was not commonly applied until 1921, when folklorist Margaret Murray promoted the notion of covens being made up of 13 witches.
Although covens of yesteryear probably met in secret locations due to the continual misunderstanding of those who practice non-traditional religions (a behavior that might get them burned at the stake,) today's witches have the option of joining an online coven. Most internet covens have membership numbers of 100 or less, and many of them are subscription based.