The definitions are approximate, but this is how I understand the terms, and how I use them. A soup is a liquid that contains stuff such as meat or fish, vegetables, and spices. A chowder is thicker, such as with the addition of potatoes and/or milk. For most people a chowder has to contain milk, but technically it doesn’t. For instance, Manhattan clam chowder is a a clam soup with a tomato sauce base and doesn’t contain milk at all. A stew I define as a soup but with much less water, often cooked longer than a soup. So, there you have it.