In the 'real' world:
A Wizard is commonly considered a person who seeks to master the arts of magic. Generally speaking, Wizards are positive practitioners of magick.
‘Wizard’ derives from the Middle English ‘wis’, meaning ‘wise’. The word first appears about 1440, meaning a ‘wise man or woman’; in the sixteenth and seventeenth centuries it designated a high magician, and only after 1825 was it used as the equivalent of ‘witch’.”
According to common understanding, the word "Warlock" comes from the Old English word ‘waerloga’, meaning "oath-breaker". It is thought to be a term applied to a person who broke the oath of secrecy to his/her clan, or group {or coven}. A person who broke their oaths and was found faithless was cast out, and black-listed. A warlock is currently considered a very bad thing to be. Calling a male Witch a warlock is often seen as a very grave insult to the integrity of the Witch.
Therefore, Voldemort is a Warlock, not a wizard.
