A viscount is a noble who is ranked higher than a baron but below a count. Traditionally, they are the son or younger brother of a count.
In England, the title was also used after the Norman conquest for the Shire Reeve or sheriff. A viscountess is a female viscount.
Formally the English viscounties were first created in 1440, but the title existed (as vice-comte or deputy-count) into the days of the Carolingian empire.
Viscounts in the SCA
In the SCA, a viscount is a person who has ruled a principality at least once. They are referred to as "Your Excellency" and wear a coronet of their choice, most commonly with pearls. However, in the Middle Kingdom and its descendant kingdoms (Calontir, Ealdormere and Northshield), viscounts wear embattled silver coronets.