在数学上，对角线有多个定义：

在多边形上，对角线是链接两个不毗邻顶点的线段。若且唯若多边形的对角线在多边形之内，该多边形是凸多边形。

对角线数目的求法：当n是多边形的顶点的数目，每个顶点除了它本身和毗邻的两边之外，都对角线连接到其他顶点。这样乘以顶点的数目就是。因为每个顶点计算了两次，所以再除以2就得出对角线的数目为

In geometry, a diagonal is a line segment joining two vertices of a polygon or polyhedron, when those vertices are not on the same edge. Informally, any sloping line is called diagonal. The word "diagonal" derives from the ancient Greek διαγώνιος diagonios, "from angle to angle" (from διά- dia-, "through", "across" and γωνία gonia, "angle", related to gony "knee"); it was used by both Strabo and Euclid to refer to a line connecting two vertices of a rhombus or cuboid, and later adopted into Latin as diagonus ("slanting line").