A highball is not a particular drink, it is a type of drink.
It is a highball glass with ice, a liquor such as whiskey, vodka, etc., and a mixer such as soda, cola, or juice. So, a gin and tonic is a highball, as is a screwdriver. Although it can be a bit vague because juice drinks, like a screwdriver or bloody-Mary, are often put in a Collins or pint glass.