A diagonal of a quadrilateral is a line segment that joins two __________ vertices of the quadrilateral.
A diagonal line is a line segment that connects the two vertices of a shape, which are not already joined by an edge. It does not go straight up, down or across.
In other words, a diagonal is a straight line that connects the opposite corners of a polygon through its vertex.
Hence, A diagonal of a quadrilateral is a line segment that joins two opposite vertices of the quadrilateral.