The main difference between a fitting and an adapter lies in their purpose and design. A fitting is a component that is used to connect two or more parts together, such as connecting two sections of tubing or attaching a hose to a fluid system. Fittings come in various types, including straight fittings, elbows, tees, cross fittings, and more. On the other hand, an adapter is specifically designed to join two parts that have different sizes or types of connections. It acts as an intermediary piece between the two components, allowing them to be connected despite their differences.