Diagonal, Slanting, Tilted
noun, な adjective, の adjective
The kanji and the word are exactly the same. That means they share meanings as well.
ななめ
A diagonal is anything that's not a right angle. Even if something is at a 七 (なな) percent angle is a diagonal, slanting, tilted angle.
水平な横線を引くように言ったのに、ちょっと斜めになっています。
I told him to draw a horizontal line, but it’s slightly diagonal.