What is the reason behind Mary I of England's nickname, 'Bloody Mary'?

Mary I, who reigned from 1553 to 1558, was a devout Catholic, and her primary goal was to reverse the English Reformation initiated by her father, Henry VIII. England had become increasingly Protestant, but Mary sought to restore Catholicism as the dominant religion. During her relatively short reign, she enacted laws to reinstate Catholic practices and, most notably, she oversaw the persecution of Protestants who refused to convert. Hundreds of Protestants were burned at the stake for heresy. This intense and brutal suppression of religious dissent is the sole reason she earned the infamous nickname "Bloody Mary." While she did imprison her half-sister Elizabeth (later Elizabeth I), and her reign was certainly fraught with political tension, it was the religious persecution that defined her legacy and led to the moniker. She did not fight in any battles.