A Circle of Power is a location that units in some missions of Warcraft II and Warcraft III had to be returned or delivered to. Their uses in Warcraft III are much more varied than Warcraft II.
The World of Warcraft version can be seen in  Minshina's Skull. Many similar circles can be seen as doodads, but the term itself has fallen out of use.
A circle of power is mentioned in [78G] Convocation at Zol'Heb.
In Warcraft I.
In Warcraft II.
The three different size of Circles used in Warcraft 3.