A | B |

Circle | The set of all points equidistant from a given point |

interior point | A point inside the circle |

exterior point | A point outside the circle |

Chord | A line segment whose endpoints lie on the circle |

Diameter | A chord that passes through the center of a circle |

Radius | A line segment that has one endpoint at the center of the circle and the other endpoint on the circle |

Tangent | A line that intersects a circle in one and only one point |

Point of tangency | The point where a line touches a circle at one point only |

Secant | A line that intersects the circle in two points |

Common internal tangent | A line that is tangent to two circles and crosses a line segement that connect the centers of the two circles |

Common tangent | A line that is tangent to two circles that are also tangent to each other |

Common external tangent | A line that is tangent to two circles but does not intersect a line segment that connects the centers of the two circles |

Concentric circles | Circles with the same center |

Central angle | An angle whose vertex lies at the center of the circle |

Arc | An unbroken part of a circle |

Minor Arc | An arc whose measure is greater than zero degrees but less that 180 degrees |

Major arc | An arc whose measure is greater than 180 degrees but less than 360 degrees |

Semicircle | An arc whose measure is exactly 180 degrees |

Inscribed angle | An angle whose vertex lies on the circle |

Sector | An area of a circle created by two radii |

Intercepted Arc | An arc lies on the circle within the two sides of an angle. |