A diagonal is a straight line that connects two non-adjacent vertices of a polygon or a polyhedron. In simpler terms, it cuts across the shape, rather than following the edges. For example, in a square, the line connecting opposite corners is a diagonal. In geometry, diagonals are important for understanding the properties of shapes. For instance, a rectangle has two diagonals that are equal in length, while a triangle has no diagonals since all its vertices are adjacent. Diagonals can also be found in graphs and matrices, where they help in analyzing relationships and data.