The "traditional" definition of a coven is a group of thirteen witches working together.
"Coven" is used by Wiccans to refer to a congregation that worships and performs workings together, with a High Priestess and/or a High Priest. They are usually limited to 13 members or less, usually not because of mystical reasons but rather practicality -- 13 people in a Circle is pretty big (it's worse if said coven is attempting to practice in someone's living room). Members of a coven are usually (whether originally or by design) close friends, particularly with the High Priestess/Priest.
Coven members usually are from the same tradition, with a common Book Of Shadows between all of them.
The Wicca use of coven leads to the term "covener", which is a member of a coven and therefore the opposite of a solitary witch. Coveners have the advantage of having a pagan social group/support group right with them, but on the other hand must compromise with others and live with them as well.
In the past, magical and initiatory training were emphasized for the High Priestess/Priest, but in recent years, a new emphasis on skills such as counseling and conflict resolution that more mainstream ministers should possess has emerged.