A more thorough explanation:

Definition: A viscountess is the wife of a viscount. She holds the title of viscountess.

For example, Lady Mary is the viscountess of the town. Her husband, Lord John, is the viscount of the town. Lady Mary is addressed as "Viscountess" instead of "Lady" because of her husband's title.

Another example is Lady Jane, who is married to Viscount William. She is referred to as Viscountess William.

These examples illustrate that a viscountess is a woman who is married to a viscount and holds the title of viscountess. The title is used to address her instead of "Lady" or her first name.