A magnate in the Kingdom of Valyria is a member of the highest levels of nobility, specifically a duke or a count. These nobles are called “peers” of the King because of their status as holders of vast swaths of land. In fact, the meaning of “prince” only refers to the King’s status as “first” or “principal” among equals. Magnates are important in Valyria also because they each have the right to a seat in the House of High Lords.
Currently, barons are not considered magnates, although they are considered lords. There is currently a movement in Valyria that is encouraging the creation of the title viscount in Valyria. Viscounts would be allowed in “grand” counties composed of multiple shires. Under the proposition, a viscount would also be allowed a vote in Parliament as a magnate.