Definition of "usurper"

The fact that the Yongle emperor was therefore a usurper, regicide and nepoticide (nephew-killer) made compiling the Veritable Record – or official history – of his reign a most dangerous scholarly post. The official given this ticklish task managed to survive several drafts, finally producing one that pleased his master as it omitted the dead nephew's reign altogether.

2014 September 15, Martin Gayford, “There's more to Ming than a vase ”, in The Daily Telegraph (Review)‎