For American Airlines passengers, "changing terminals" means that your connecting flight departs from a different building or terminal area than the one you arrived in. This is most common at large hub airports like Dallas/Fort Worth (DFW), Chicago O'Hare (ORD), or Los Angeles (LAX). At DFW, for example, American operates out of Terminals A, B, C, D, and E. If you arrive at Gate A20 and your next flight is at Gate D15, you must "change terminals." In most major AA hubs, you can use a post-security shuttle or train (like the "Skylink" at DFW) to transfer between terminals without having to exit the secure area or go through TSA security again. However, if you are arriving on an international flight, you must always clear customs and immigration, which often involves exiting the secure terminal, reclaiming your bags, and then re-clearing security at your departure terminal. It is vital to check your boarding pass or the American Airlines app for the "Terminal" and "Gate" letters to ensure you have enough "Minimum Connection Time" for the transfer.