- adjective referring to a line, plane or structure that slopes halfway between the vertical and horizontal
- noun a line joining two opposite corners of a rectangle
- A straight structural member forming the hypotenuse of a right triangle, as in the diagonal bracing of a stud wall.
- In a matrix, the elements on a straight line from the top left to the bottom right, or occasionally from the bottom left to the top right.
- In an Edgeworth box, the straight line from the bottom left corner to the top right. Along the diagonal, the ratios of allocations of the two agents (industries or consumers) are constant and equal.
- In an integrated world economy diagram, the straight line from the bottom left corner to the top right. Along the diagonal, the ratios of factor endowments of the two coutries are constant and equal.
Origin & History of “diagonal”
Diagonal is commonly used simply as a synonym for oblique, but in strict mathematical terms it denotes a line joining two non-adjacent angles of a polygon. this reveals far more clearly its origins. It comes from diagōnālis, a Latin adjective derived from Greek diagṓnios. This was a compound formed from the prefix dia- ‘across’ and gōníā ‘angle’ (as in English polygon), meaning ‘from angle to angle’. Gōníā is related ultimately to English knee and genuine.