An external domain is an existing active domain that is not registered with easyname but with another domain provider.
With easyname you have the option to integrate an external domain into your Control Panel free of charge. You can then use your Domain with us, although it is registered with another provider.
In contrast to a Transfer, your current provider remains the same and your provider is still responsible for your Domain.
A step-by-step guide to add your domain as an external domain can be found here.