A kind of ice-cream soda, made with lemonade (the Sprite and 7-Up sort, not the kind with lemons), and ice-cream topping. Generally served in a big glass, about the height of a large milkshake from McDonald's (I call them spider glasses, but this might be a bit of a circular definition.)
Ice-cream sodas are often also called spiders, but these can contain any soft drink, and don't have the topping.