It's a straight line for example within a square that joins opposite corners
diagonal
It looks like this////
Diagonal LineA diagonal line goes from one corner to another corner. See picture at the Related Link.
red flag with white diagonal stripe
An equals sign with a diagonal line through it ≠
"Diagonal" is not a shape, it is a direction, or orientation. If you drew a line from one corner of a square to the opposite corner, that new line would be diagonal to the square. Here is an example: / or \
two diagonal lines that aren't touching
red flag with a diagonal white stripe
An equals sign with a diagonal line through it.
Parallelogram
The bishop has two diagonal cuts for two reasons. First it is to look like a bishop's Mitre, his ceremonial hat which has diagonal sides. The second is to show that the Bishop moves in diagonal directions.
"Diagonal" is not a shape, it is a direction, or orientation. If you drew a line from one corner of a square to the opposite corner, that new line would be diagonal to the square. Here is an example: / or \