a four-sided polygon | quadrangle; quadrilateral; tetragon; |

a three-sided polygon | triangle; trigon; trilateral; |

something approximating the shape of a triangle | triangle; |

a triangle whose interior angles are all acute | acute-angled triangle; acute triangle; |

a three-sided regular polygon | equiangular triangle; equilateral triangle; |

an object shaped like an equilateral triangle | delta; |

a triangle with two equal sides | isosceles triangle; |

a triangle that contains no right angle | oblique triangle; |

a triangle that contains an obtuse interior angle | obtuse-angled triangle; obtuse triangle; |

a triangle with one right angle | right-angled triangle; right triangle; |

a triangle with no two sides of equal length | scalene triangle; |

a regular polygon formed by extending each of the sides of a regular hexagon to form two equilateral triangles | hexagram; |

(mathematics) one of a set of parallel geometric figures (parallel lines or planes) | parallel; |

a quadrilateral whose opposite sides are both parallel and equal in length | parallelogram; |

a quadrilateral with no parallel sides | trapezium; |

a quadrilateral with two parallel sides | trapezoid; |

a plane figure with 5 or more points; often used as an emblem | star; |

(mineralogy) a star-shaped figure with six rays that is seen in some crystal structures under reflected or transmitted light | asterism; |

a star with 5 points; formed by 5 straight lines between the vertices of a pentagon and enclosing another pentagon | pentacle; pentagram; pentangle; |

a five-sided polygon | pentagon; |