SOAP
Simple Object Access Protocol — an XML-based messaging protocol for web services, standardized at the W3C in 2000, foundational to the WS-* enterprise stack.
SOAP (originally Simple Object Access Protocol; the acronym was dropped in version 1.2) is an XML-based protocol for exchanging structured messages, typically over HTTP or SMTP. Microsoft, IBM, and others drove it through the W3C, where SOAP 1.1 became a Note in 2000 and SOAP 1.2 a Recommendation in 2003. A SOAP message is an XML Envelope containing an optional Header (for routing, security, transactions) and a mandatory Body (the payload or fault). Service contracts are described in WSDL, and a sprawling family of WS-* specifications — WS-Security, WS-Addressing, WS-ReliableMessaging, WS-Policy, WS-AtomicTransaction — layer enterprise features on top. SOAP genuinely excels at formal contracts, transport neutrality, message-level security, and reliable delivery, which is why it remains common in banking, telecom, and government B2B integrations. But the same machinery is heavy for ad-hoc public APIs, and its verbosity, tooling dependence, and steep learning curve drove most public-facing services to REST API and JSON between roughly 2008 and 2015.