A viscount is a member of the British peerage ranking above a baron and below an earl. A viscount is said to hold a "viscounty" or "viscountcy".
The eldest son of an Viscount generally has the courtesy title of Lord, younger sons are known as the Honourable [Firstname] [Lastname] and daughters are known as the Honourable [Firstname] [Lastname].
The word derives from the Latin vice comes which was the rank of a deputy to a count.
--
The same or a similiar rank exists in other monarchies.