a double curve resembling the letter S | s-shape; |

the curve theoretically assumed by a perfectly flexible and inextensible cord of uniform density and cross section hanging freely from two fixed points | catenary; |

the double curve of the upper lip when considered to resemble Cupid's bow | cupid's bow; |

an undulating curve | undulation; wave; |

the exterior curve of an arch | extrados; |

something in a thin curved form (like the neck of a goose) | gooseneck; |

the interior curve of an arch | intrados; |

a circular segment of a curve | bend; crook; turn; twist; |

a sharp curve or crook; a shape resembling a hook | crotchet; hook; |

(biology) any hook-shaped process or part | uncus; |

a curve that is tangent to each of a family of curves | envelope; |

a bend or curve (especially in a coastline) | bight; |

a line traced by a point traveling in a constant direction; a line of zero curvature | straight line; |

(mathematics) the shortest line between two points on a mathematically defined surface (as a straight line on a plane or an arc of a great circle on a sphere) | geodesic; geodesic line; |

a straight line at right angles to another line | perpendicular; |

a connecting shape | connection; connexion; link; |

a straight line that is the limiting value of a curve; can be considered as tangent at infinity | asymptote; |

a straight line or plane that touches a curve or curved surface at a point but does not intersect it at that point | tangent; |

a straight line that intersects a curve at two or more points | secant; |

a line enclosing a plane areas | perimeter; |