Short for Enterprise Service Bus, also referred to as a message broker. ESB is an open standards-based distributed synchronous or asynchronous messaging middleware that provides secure interoperability between enterprise applications via XML, Web services interfaces and standardized rules-based routing of documents. In practice, this means that data files are passed to and from their destinations based on pre-established guidelines that are common to all parties sharing the information to ensure that the data maintains its integrity as it is routed. The multi-language and multi-platform design of an ESB allows enterprises to process data between applications from various sources. Two common distributed computing architectures used by ESBs are J2EE and .NET.

ESB is an extension of EAI, an earlier form of middleware, but ESB adds several key functions: