Refers to the formal delivery of a legal document (or instrument) to a person or company. Service might for example be of a complaint or summons initiating a proceeding (sometimes called a “process”), a court order, a subpoena or other type of important legal document. Because courts and the law are concerned that the recipient of such a document understand that it is important and it has legal consequences, service is usually required to be done with some ‘ceremony,’ i.e., in such a way that the recipient knows that they have been served and that there is a record of service. Professional service agents may for example try to take a photo of the recipient with the document in their hand. Usually courts will seek an “affidavit of service,” i.e., a sworn statement from the server that they carried out the service in manner specified in the applicable law. International service is carried out under the rules specified in the Hague Service Convention.
In the US it is not uncommon to hire professional process servers – whose jobs may be prosaic, or they may be very difficult, if a recipient knows service is coming and seeks to dodge service.