From Chanticleer Society

Spirit + sugar + mint served over finely crushed ice

The julep has gone through a variety of transformations over the centuries, originally the term referred to a strictly medicinal potion, but in the American South, it was used to describe a heavily iced beverage that added sugar and mint to a spirit. Just as a cocktail can be described as a “bittered sling”, a julep can be described as a “minted sling”.

For an example, see: