A widow is a woman whose husband has died. If your uncle dies, your aunt will become a widow.
If a person's spouse dies, that person is referred to as either a widow or — in the case of a man whose wife dies — a widower. In most cases, people stop using the word widow if the woman marries again. Back in the sixteenth and seventeenth centuries, it was common to use widow as a prefix to a woman's name, instead of "Mrs." The Indo-European root word of widow means "be empty."