Well, an ICBM is an intercontinental ballistic missile (just thought you would like to know that), and Intel defines the rest.
The IPMI Initiative consists of three specifications: Intelligent Platform Management Interface, Intelligent Platform Management Bus (IPMB) and the Intelligent Chassis Management Bus (ICMB). The IPMI Initiative derives its name from the main specification (IPMI) which defines the messages and system interface to platform management hardware. The IPMB specification defines an internal management bus for extending platform management within a chassis, while the ICMB specification defines the external management bus between IPMI enabled systems.
EDIT: typo