It's simply when your phone disconnects from your current carrier and attaches to a new network. International roaming happens when your phone disconnects from your current carrier while you're out of the country. Domestic roaming happens when it disconnects while in the U.S. (or U.S. territories).

The term "roaming" generally refers to using your mobile device on a network other than your "home" network, while "international roaming" specifically refers to using your phone in a foreign country. In a domestic context, roaming used to occur when you traveled to a rural area where your primary carrier didn't have towers, causing your phone to "roam" onto a partner's signal—often at no extra charge today. International roaming, however, is much more complex and expensive. When you cross a national border, your phone connects to a local carrier (e.g., Vodafone in the UK while you have a T-Mobile US plan). Unless you have a specific international plan, you can be charged exorbitant "pay-as-you-go" rates for every megabyte of data, every minute of talk, and every text message. In 2026, many carriers offer "Travel Passes" (typically $10/day) that allow you to use your home data bucket abroad. For travelers, the best way to avoid high international roaming fees is to either use an eSIM from a provider like Airalo or to purchase a local physical SIM card upon arrival, which provides local data rates that are significantly cheaper than standard roaming packages.