in a straight and sloping way that is not horizontal or vertical, for example joining two opposite corners of a square or other flat shape with four sides

It's quickest if you cut diagonally across the park. 从公园斜穿过去最快。

Peel and slice the carrots diagonally into half-inch slices. 胡萝卜去皮，斜切成半英寸长的段块。