Definition from Nolo’s Plain-English Law Dictionary
Serving a summons or other legal document in a lawsuit on a defendant by publishing the document in an advertisement in a newspaper of general circulation. Service by publication is used to attempt to notify a defendant who is intentionally absent, in hiding, or at an unknown address. It's allowed only after a judge has been convinced, based on a sworn declaration, of the serving party's inability to find the defendant after trying hard. Service by publication is commonly used in a divorce action to serve a spouse who has disappeared without a leaving a forwarding address or to give notice to people who might have a right to object to a "quiet title" action to clear title to real estate.