Polar coordinates
A point described by its distance from a centre and its angle around it.
See it in Fractal mirror. Pick Kaleidoscope under Mode: one slice of the picture is repeated by angle all the way round.
In plain words
Most grids name a point by how far across and how far down it is. Polar coordinates name it another way: how far from a centre, and at what angle. A clock hand works like this, with its length and the hour it points to. Circles, spirals and spinning patterns are easy to describe this way. A kaleidoscope reads each pixel's angle and repeats the same slice round the centre.