A device that makes a very clear and theoretically uncrossable demarcation between Inside and Outside. Done with much ornate ceremony by wizards, chanting and occultism by young and impressionable wannabe-witches such as Diamanda Tockley, and by scuffing her boot around in the dust to make a rough circle by Nanny Ogg. However it's done, it needs to be pretty good to keep all the serious magic inside and have none of the effects get to the outside. Anyone inside the circle is fair game for whatever turns up.
A Magic Circle has also been used creatively, ie to allow Rincewind and Eric to remain in exactly the same place while aeons, or maybe even Ians, of time and evolution pass by on the other side of a very thin chalk line. This also allows them to evade the personal attentions of a very irritated King of the Demons called Astfgl.
The Magic Circle is a British organisation dedicated to promoting and advancing the art of magic. The motto of the society is the Latin indocilis privata loqui, which the club claims to mean "not apt to disclose secrets"