Warlock
For other meanings of the term, see warlock (disambiguation).
Warlocks are, among historic Christian traditions, said to be the male equivalent of witches (usually in the pejorative sense of Europe's Middle Ages), and were said to ride pitchforks (six-foot [~1.83 m] two-tined forks, for manoeuvring unbound hay) instead of broomsticks. In some pop culture TV shows like Buffy the Vampire Slayer and Bewitched, the case is that warlocks are male witches. The TV show Charmed is more accurate to Wicca, wherein a warlock just means "Traitor to an oath". They can be male or female just like a witch (good or neutral). By this definition, a warlock is an evil witch.
A possible origin
However, this may be a new meaning, as the frequent use of "warlock" to describe a male witch is largely based on Hollywood scriptwriters, especially those writing for the 1960s sitcom, Bewitched1.
Among a few traditions of neo-pagans, a "warlock" is a punishment ceremony, not a person. In the ceremony a person's access to magical power is "locked" and he or she has no ability to perform effective magical spells, or have access to powers at all. This is regarded, by those who believe in it, as one of the most severe punishments that can be meted out among the neo-pagan community.
Many neo-pagans consider the word an insult, probably because of its etymology. The word itself comes from a Scottish word meaning "oathbreaker" or "liar"2. However, one source suggests that the word may come from the Old Norse Vard-lokkur, "caller of spirits".
Other possible origins
There is another, folk-etymology version for the origin of the word warlock, coming from Old English 'wær-loga', the man of the logs, alluding to the small pieces of wood the Anglo-Saxon and Scandinavian priests and wise men used to divine by means of the runes. This seems to have been a slang word of Christian coinage pejoratively used for those who remained Pagan and practising the art of the runes. By extension, it became a synonym of sorcerer and wizard, and also of a typical medieval diabolical male witch (in this sense either able to fly in several ways, see Sabbath, witchcraft). The use of the word witch to name both witch and warlock is turning the word into an archaic one.
Warlock (British) also means 'oath-breaker'.
References
1: Pavlac, Brian A. "10 Common Errors and Myths about the Witch Hunts, Corrected and Commented," Prof. Pavlac's Women's History Resource Site. (October 31, 2001) [1] (October 8, 2003).