The term “warlock” is a very old one. Although it is sometimes used as interchangeable with “wizard”, it originally denoted one learned in duelling and all martial magic. It was also given as a title to wizards who
had performed feats of bravery, rather as Muggles were sometimes knighted for acts of valour (WHH).
Definition
Warlock is an ancient wizarding term originally used to denote a wizard who is trained in martial skills such as dueling. The meaning has been diluted somewhat over the centuries and is now used for a wizard who looks particularly fierce or as a title of achievement (WHH).
Commentary
Notes
In her commentary in The Tales of Beedle the Bard, Rowling describes the term "warlock" in this way:
The term “warlock” is a very old one. Although it is sometimes used as interchangeable with “wizard”, it originally denoted one learned in duelling and all martial magic. It was also given as a title to wizards who had performed feats of bravery, rather as Muggles were sometimes knighted for acts of valour ... These days wizards use “warlock” in one of two ways: to describe a wizard of unusually fierce appearance, or as a title denoting particular skill or achievement ... (WHH)
From the Web
"Warlock" on the Harry Potter Wiki