A social circle on the Internet.

A phenomenon that occurs on rivers during winter, where floating ice caught in an eddying flow will rotate around, knocking off its distant edges, forming a floating revolving circular disk, slowing turning under the influence of the underlying vortex current; when the temperature conditions are not too warm, leading to ice breakup and fragility, and not too cool, freezing over the entire surface.

(astronomy) The great circle through an object whose location is to be determined and the two celestial poles.

(of a black hole) Synonym of photon sphere

celestial equator

(algebra) The group of all complex numbers of unit modulus, with the group operation being complex multiplication.

(birdwatching) A designated radius in which bird populations are measured as part of a census.

Synonym of fairy ring (“ring of fungi”)

(geometry) A circle that cuts three other given circles at right angles.

Any of various games for groups of children where players sit or stand in a circular formation.