What does the term ESB- Enterprise Service Bus- actually mean? That question has been the topic of an ongoing debate for several years now that doesn't seem to have any sign of stopping. When I first read about ESBs in 2003, I didn't expect to still be trying to understand them more than four years later. In comparison, there have been other technologies that in the same time frame were introduced, developed, matured, and obsolete. However, it has felt like the definitions are getting closer in spirit if not words. Here is a current selection that I could find (using search to find the pages that were most referenced but possibly not most recent).

BEA defines the ESB:

At the highest level, an ESB provides common communication and integration services for composite applications and shared services in an SOA.

IBM defines the ESB:

An enterprise service bus (ESB) is a pattern of middleware that unifies and connects services, applications and resources within a business.

Oracle defines the ESB:

It provides a much-needed intermediary layer that facilitates data delivery, service access, service reuse, and service management of an enterprise SOA implementation.

Sonic defines the ESB:

An ESB is software infrastructure that simplifies the integration and flexible reuse of business components within a service-oriented architecture.

Tibco defines the ESB:

An enterprise service bus (ESB) is a standards-based communication layer in a service-oriented architecture (SOA) that enables services to be used across multiple communication protocols.

Microsoft has a site that is being put together to provide ESB guidance but dodges the question of definition (the site plays up the ESB means different things to different people angle).

At the end of the day though, the clearest definition of what companies think ESB means comes from looking at the products that they build.