未亡人 miboujin

I think it is all right because it is the simplest words.

There is some other way of saying.

わたしは夫と死別しました watashi ha otto to shibetsushi mashita

わたしの夫は亡くなりました watashi no otto ha nakunari mashita

わたしは夫に死に別れました watashi ha otto ni shiniwakare mashita