A "diagonal" is a straight line that connects two non-adjacent corners of a shape, such as a rectangle or a square. In geometry, it helps to divide shapes into smaller parts and can be used to calculate distances and areas. Diagonals are important in various fields, including architecture and design, where they can create visual interest. In a grid or matrix, diagonals can refer to lines that run from one corner to the opposite corner. For example, in a chessboard, the diagonals are the lines that connect squares of the same color. Understanding diagonals is essential for solving problems related to angles and symmetry.