circle has 18 distinct synonyms spread across 7 of its 11 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
ellipse in which the two axes are of equal length; a plane curve generated by one point moving at a constant distance from a fixed point
“he calculated the circumference of the circle”
an unofficial association of people or groups
“the smart set goes there”
something approximating the shape of a circle
“the chairs were arranged in a circle”
movement once around a course
“he drove an extra lap just for insurance”
a road junction at which traffic streams circularly around a central island
“the accident blocked all traffic at the rotary”
street names for flunitrazepan
a curved section or tier of seats in a hall or theater or opera house; usually the first tier above the orchestra
“they had excellent seats in the dress circle”
any circular or rotating mechanism
“the machine punched out metal circles”
travel around something
“circle the globe”
Synonyms, antonyms and the broader and narrower relations for circle come from Princeton University’s WordNet. Full sources and licences.