noun a member, in a framed structure, running obliquely across a panel.
2. diagonal ·noun a diagonal cloth; a kind of cloth having diagonal stripes, ridges, or welts made in the weaving.
3. diagonal ·noun a right line drawn from one angle to another not adjacent, of a figure of four or more sides, and dividing it into two parts.
4. diagonal ·adj joining two not adjacent angles of a quadrilateral or multilateral figure; running across from corner to corner; crossing at an angle with one of the sides.