Prince or King of Yan was a Chinese feudal title referring to the ancient Chinese State of Yan and to its fiefs including the capital Yanjing (located within modern Beijing).
Typically, the title is translated as "King of Yan" for rulers prior to the establishment of the Chinese empire by Shi Huangdi and "Prince of Yan" as a peerage title after the restoration of feudal titles by the Han dynasty. It was generally held by powerful members of the imperial family and owing to its important position protecting central China from Mongolian and Manchurian invaders typically included powerful and well-fortified military forces.
More specifically, "Yanwang" in Chinese and "Prince of Yan" in English typically refers to Zhu Di, who held that rank before launching the Jingnan Campaign which established him as the Ming dynasty's Yongle Emperor in the 15th century.