A dessert made of ice cream in a dish with things poured or piled on top — chocolate or strawberry sauce, nuts, whipped cream, fruit, often a cherry. The key idea is that it is ice cream *dressed up* and eaten with a spoon from a bowl or a tall glass, not a cone you hold. It is an old-fashioned, cheerful, American-diner sort of word, and in Taiwan most students meet it through fast food — the McDonald's 聖代 is exactly this word. It is countable: 'two chocolate sundaes, please'. The spelling is strange, and that is the whole story of the word: it comes from 'Sunday', respelled so as not to look like the holy day was being sold in a shop. Where and why it started is argued over — several American towns claim it — so I would not tell you any one of those stories as fact.