A warlock is a man with magic powers, usually the dark or evil kind. He belongs to stories, films, and games — fantasy worlds, old folk tales, Halloween. People often teach it as "the male witch", and that is a fair first answer, but it is not quite true, and the difference is worth knowing. "Witch" itself has never been only for women: a man can be a witch, and men who practise witchcraft today usually call themselves witches, not warlocks. Many of them dislike "warlock" because of where the word comes from — its old meaning was traitor, oath-breaker. So in real modern use, warlock mostly lives in fiction. In fantasy games it has become a job title: a warlock is a character class who gets power by making a deal with some dark being, which fits the old sense of the word rather neatly. It is a normal countable noun — a warlock, two warlocks.