A circle of mirrors was a series of mirrors, or reflective surfaces, arranged in a circle for unique purposes.
Using a circle of mirrors created from reflective solar panels on Deva Loka, the Fifth Doctor and the Kinda were able to free Aris from the possession of the Mara, as it could not stand it's own reflection because it was evil. Keeping the circle closed, the Mara grew rapidly, expending its energy seemingly in an effort to escape, but was eventually dispersed back to the Dark Places of the Inside, or "wherever." (TV: Kinda) The Mara was later able to re-possess Tegan Jovanka and spoke to her in a hall of mirrors on Manussa. When Tegan stated that the Mara was repelled by mirrors, it clarified that was repelled by a circle of mirrors, and that there was no circle in the hall. (TV: Snakedance)
In the alternate timeline of Donna's World, Rose Tyler and UNIT utilised the Doctor's TARDIS and a circle of mirrors to reveal to Donna Noble the Time Beetle that was on her back. They later used the circle to send her back in time to negate the incorrect timeline. (TV: Turn Left)
In similar principal to a "circle," Theodore Maxtible used 144 separate polished mirrors in a closed space and applied static electricity to achieve a form of projected time travel. (TV: The Evil of the Daleks)