a straight line from the center to the perimeter of a circle (or from the center to the surface of a sphere) | radius; |

a straight line connecting the center of a circle with two points on its perimeter (or the center of a sphere with two points on its surface) | diameter; |

a line that bisects a plane figure | center line; centerline; |

a concave shape whose distinguishing characteristic is that the concavity faces downward | dome; |

a concavity in a surface (especially an anatomical depression) | fossa; pit; |

a small concavity | corner; niche; recess; recession; |

a shape whose base is a circle and whose sides taper up to a point | cone; cone shape; conoid; |

a conical shape with a wider and a narrower opening at the two ends | funnel; funnel shape; |

(geometry) a curve generated by the intersection of a plane and a circular cone | conic; conic section; |

a point or set of points common to two or more geometric configurations | intersection; |

a plane figure that deviates from a square or circle due to elongation | oblong; |

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 | circle; |

a small circle | circlet; |

something approximating the shape of a circle | circle; |

a circle dividing a sphere or other surface into two usually equal and symmetrical parts | equator; |

a plane figure with the shape of half a circle | hemicycle; semicircle; |

a continuous portion of a circle | arc; |

one of a series of rounded projections (or the notches between them) formed by curves along an edge (as the edge of a leaf or piece of cloth or the margin of a shell or a shriveled red blood cell observed in a hypertonic solution etc.) | crenation; crenature; crenel; crenelle; scallop; |

a straight line connecting two points on a curve | chord; |

a plane figure bounded by two radii and the included arc of a circle | sector; |