(sports, boxing, wrestling, professional wrestling) The boxing or wrestling ring.

(geometry) A circle of radius 1. / (trigonometry, the unit circle) The circle of radius 1 with centre at the origin, used in defining trigonometric functions.

(soccer) The circle in the centre of the pitch which, at kick-off, the defending team may not enter until the attacking side has touched the ball.

(baseball) The circle drawn or painted on the field where the next batter to hit waits.

(automotive, of a road vehicle) The circular path formed by the wheel furthest from the centre of the turn when turning to the maximum extent.

(meteorology) A luminous halo, each side of the sun parallel to the horizon, caused by refraction of sunlight through ice crystals.

A supposed error in reasoning attributed to René Descartes. A form of circular argument.

plural of Cartesian circle

plural of crop circle

A situation in which the solution to one problem makes each future problem easier to solve than the original one.