Diagonal (adjective)

1. Lying, extending, or inclining from one corner or angle of a square or rectangular figure to another diagonal corner or angle.
2. Joining two opposite corners of a square or rectangular figure.

Origin:

Late 16th century: from french diagonale, or via latin from greek diagonios 'from angle to angle', from dia 'across' + gonia 'angle'.

Examples:

1. She measured the length of the diagonal of the square.
2. The diagonal cut across the room from one corner to the other.
3. The diagonal line on the graph showed an upward trend.
4. The diagonal of a cube is the longest distance between two opposite vertices.
5. The curtains had a diagonal pattern that added a modern touch to the room.