Ser'ing is converting Java objects into (or from) a binary stream. It is defined in Java terms.
XML is a platform-independent textual format, in harsh contrast to the Java-specific binary ser'ed format.
In XML, "serialization" usually means converting some internal format into standard XML text. This internal format is often a DOM, but it certainly isn't limited to that. You could have an organized collection of Java objects along with code with converted the collection into an XML document which represented it, and you would call that "serialization".