A woman who has lost her husband; generally means a widow (sometimes used in legal/official contexts to refer to divorced or unmarried women in certain senses).
戦争で夫を失い、彼女は寡婦になった。
She lost her husband in the war and became a widow.
その法律は寡婦に対する年金について規定している。
The law provides rules regarding pensions for widows.