Domain hijacking is when someone takes your domain away from you. Usually, it is accomplished by falsifying a domain transfer authorization.

The best way to prevent your domain from being hijacked is to keep it locked. You can lock all your domains from your Dynadot control panel for free, and in fact, we make this the default setting when you register or transfer a domain to us to ensure domain security. When a domain is locked, a domain transfer cannot even be started by another Registrar.

To read about the most famous domain hijacking case, read the Wikipedia entry for Sex.com.