Coldwater fish are generally considered as those fish that do not need a heater.
This is a little vague however, and is a more accurate description of temperate fish. An unheated aquarium in a typically heated room will settle at between 18-22C (64-71F), which is the temperature realm inhabited by temperate fish including many species often sold as tropical fish.
True coldwater fish can withstand temperatures down to around 2C in the winter, when they enter a period of dormancy and head down to the deeper regions where the water is warmest.
Under normal conditions, true coldwater fish are found at temperatures between 10-18C (50-64F) although most will tolerate the warmer waters experienced in unheated indoor aquariums.