Mizuwari means "mixed with water" and refers to a popular way of drinking spirits in Japan. As a style of drink, it usually consists of two parts of cold water mixed with one part of the spirit and lots of ice. While the practice comes from Japanese shochu drinking traditions it is a very popular way of drinking whisky.
If you go to a Japanese cocktail bar and ask for a whisky as mizuwari, they'll know how to make it and the concept has now started to travel around the world. If you ask for a whisky highball it’ll be served with soda, ask for it as a Mizuwari it will come with flat water. Both will likely be served in tall glasses with lots of ice and make for a refreshing drink!