"An XML-based protocol for invoking remote procedures. SOAP has become one of the cornerstones of Web services, and has been accepted as a standard by all major vendors including Microsoft, SUN, IBM, and others." (Atul Apte, "Java™ Connector Architecture: Building Custom Connectors and Adapters", 2002)
"A standardized protocol used to call Internet-based services (Web services) by exchanging XML messages." (Johannes Link & Peter Fröhlich, "Unit Testing in Java", 2003)
"Designed by the World Wide Web Consortium (W3C), this transport protocol provides a standard way to send messages between applications using XML." (Sara Morganand & Tobias Thernstrom , "MCITP Self-Paced Training Kit : Designing and Optimizing Data Access by Using Microsoft SQL Server 2005 - Exam 70-442", 2007)
"is a protocol for exchange of information in a distributed environment. It is an XML based protocol consisting of three parts: an envelope (a framework for describing what is in a message and how to process it), a set of encoding rules (for expressing instances of application-defined datatypes), and a convention for representing remote procedure calls and responses." (Craig F Smith & H Peter Alesso, "Thinking on the Web: Berners-Lee, Gödel and Turing", 2008)
"A protocol for exchanging XML messages over a network. It defines the structure of the XML messages (the SOAP envelope), and a framework that defines how these messages should be processed by software." (Mark Olive, "SHARE: A European Healthgrid Roadmap", 2009)
"A wrapper specification from the World Wide Web Consortium (W3C) for requests for web services that facilitates interoperability between a broad mixture of programs and platforms." (DAMA International, "The DAMA Dictionary of Data Management", 2011)
"A protocol specification for exchanging structured information in a Web services implementation." (Craig S Mullins, "Database Administration", 2012)
"A protocol specification for exchanging data. Along with REST, it is used for storing and retrieving data in the Amazon storage cloud." (Marcia Kaufman et al, "Big Data For Dummies", 2013)
"A common protocol used to establish a session between a Web server and mobile app; it is stateless by design." (Mike Harwood, "Internet Security: How to Defend Against Attackers on the Web" 2nd Ed., 2015)
"A lightweight protocol for exchange of information in a decentralized, distributed environment." (Adam Gordon, "Official (ISC)2 Guide to the CISSP CBK" 4th Ed., 2015)
"A standard format in XML for sending data between a client and a Web service. The service's WSDL file specifies the format of the SOAP messages it expects and will return." (Microfocus)