A more thorough explanation:

A widow is a person whose spouse passed away during their marriage and has not remarried since then. This definition is according to the United States Copyright Law, 17 USC § 101.

After her husband's death, Mary became a widow. She did not remarry and continued to live alone.

Another example could be:

John's wife passed away last year. He is now a widow and is still grieving her loss.

These examples illustrate the definition of a widow. In both cases, the person's spouse died during their marriage, and they did not remarry. Therefore, they are considered widows according to the legal definition.