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。

Reflection symmetry, line symmetry, mirror symmetry, mirror-image symmetry, or (in biology) bilateral symmetry is symmetry with respect to reflection. That is, a figure which does not change upon undergoing a reflection has reflectional symmetry.

In 2D there is a line of symmetry, in 3D a plane of symmetry. An object or figure which is indistinguishable from its transformed image is called mirror symmetric.