轴对称或线对称指一个图形沿一条直线折叠，直线两旁的部分能够互相重合。更广泛的对称形式为旋转对称。

• 定理1：关于某条直线对称的两个图形是全等形。
• 定理2：如果两个图形关于某条直线对称，那幺对称轴是对应点连线的垂直平分线。
• 定理3：两个图形关于某条直线对称，如果他们的对称轴或延长线相交，那幺交点在对称轴上。
• 定理2的逆定理：如果两个图形的对应点连线被同一条直线垂直平分，那幺这两个图形关于这条直线对称。

Generally, an object with rotational symmetry also known in biological contexts as radial symmetry, is an object that looks the same after a certain amount of rotation. An object may have more than one rotational symmetry; for instance, if reflections or turning it over are not counted. The degree of rotational symmetry is how many degrees the shape has to be turned to look the same on a different side or vertex. It cannot be the same side or vertex.