The word viscount, known to be used in English since 1387, comes from Old French visconte (modern French: vicomte), itself from Latin (“vice” meaning “deputy” + “comes” meaning “companion”).
As a rank in British peerage, it was first recorded in 1440, when John Beaumont, 1st Viscount Beaumont, was made one by King Henry VI. The word viscount corresponds in Britain to the Anglo-Saxon shire reeve (root of “Sheriff”).
A viscount is said to hold a "viscountship" or "viscounty", or (more as the area of his jurisdiction) a "viscountcy". The female equivalent of a viscount is a viscountess.
In British practice, the title of a viscount may be either a placename, or a surname, or sometimes, a combination thereof. In any event, the style of a viscount is "The Viscount X," or "The Viscount X of Y." He is addressed as "My Lord" or (more formally) "Your Lordship".
A British viscount is addressed in speech as Lord X, while his wife is Lady X, and formally styles "The Viscount X". The children of a viscount are known as The Honourable [Forename] [Surname].
A specifically British custom is the use of viscount as a courtesy title for the heir of an earl or marquess. The peer's heir apparent will sometimes be referred to as a viscount, if the second most senior title held by the head of the family is a viscountcy. For example, the eldest son of the Earl Howe is Viscount Curzon, because this is the second most senior title held by the Earl.
Biographies
I've added a couple of selected biographies - I'll be adding more when I can.
Viscount Severn
Under certain circumstances the title viscount is used as a courtesy title for the heir of an earl or marquess. This is a custom specific to England.
The most recent example of such a viscount is James Mountbatten-Windsor, Viscount Severn. James is the eighth grandchild of the Queen and Prince Phillip, the Duke of Edinburgh. He was born on 17th December 2007 to Prince Edward, Earl of Wessex and Sophie, Countess of Wessex (née Sophie Rhys-Jones).
Viscount Severn is eighth in the line of succession to the British throne and is followed by his older sister, Lady Louise Mountbatten-Windsor, who now occupies ninth place. Neither of Prince Edward’s children will bear the style HRH. A choice made my both Edward and Sophie in an attempt to shield their children from unwanted media attention. However, James’s title will remain as it is a subsidiary title of his father, the Earl of Wessex, and is also associated with the Rhys-Jones family’s Welsh connections.
