A default divorce is when a judge makes the decisions regarding a divorce after hearing only from one of the two parties. This is because the other party fails to comply with the divorce proceedings. In some circumstances, the other party may truly not be able to be found. However, the default divorce is what happens if only one party complies with the procedure.
In the event of a default divorce, the judge makes the decisions regarding whether or not the divorce should be granted, who receives custody of the children, how the property should be divided, etc., while only hearing from one of the partners in the marriage. Typically in divorce proceedings, both people present their side of the story. However, sometimes one party refuses to even take part in the process. This is why default divorces are needed.