SOAP

SOAP, the Simple Object Access Protocol, is a standard for web services messages. Based on XML, SOAP defines an envelope format and various rules for describing its contents. Seen (with WSDL and UDDI) as one of the three foundation standards of web services, it is the preferred protocol for exchanging web services, but by no means the only one; proponents of REST say that it adds unnecessary complexity. Description borrowed from the looselycoupled.com glossary.

SOAP is in wide use.