Usurper

"There are still those who call me "Usurper"."

- King Robert Baratheon

Usurper is a derogative term that refers to individuals who have seized power or lay claim to it in opposition to a "legitimate" or "rightful" ruler.

Known uses of the term

 * King Robert Baratheon is called "The Usurper" by Viserys and Daenerys Targaryen as well as by House Targaryen loyalists. The rebellion that installed him on the Iron Throne is also known as the War of the Usurper.
 * Stannis Baratheon, the actual lawful heir of the late Robert, refers to the other four Kings in the War of the Five Kings - Joffrey, Renly, Robb Stark, Balon Greyjoy - as usurpers, even Robb Stark and Balon Greyjoy who, while not laying claim to the Iron Throne, seek to establish independent kingdoms. In contrast, his brother Renly doesn't claim to be the "rightful" King.