Line Joining Two Opposite Corners of a Square

This page is the CodyCross answer for the clue below from Physics, Group 1888, Puzzle 1 (Kinetic Energy).

Question is: Line Joining Two Opposite Corners of a Square

and the answer is: DIAGONAL

Word Meaning:

DIAGONAL — (geometry) Joining two nonadjacent vertices (of a polygon or polyhedron).

Definitions from Wiktionary.