ITIL Definition: IT Infrastructure Library

IT Infrastructure Library: The collection of volumes produced by the UK Office of Government Commerce (previously CCTA) that describe IT Service Management best practice. The library is intended to assist organizations to provide quality IT service in the face of budgetary constraints, skill shortages, system complexity, rapid change, current and future User requirements and growing User expectations.

Originally produced in the late 80s - early 90s as a set of more than forty volumes, at the heart of the latest issue of the library is a set of six books:

Service Support and Service Delivery - provide advice on how to manage the core processes of IT Service Management. These are two independent volumes.
Planning to Implement Service Management - explains the steps necessary to identify how an organization might expect to benefit from ITIL, and how to set about reaping those benefits.
ICT Infrastructure Management - Network Service Management, Operations Management, Management of Local Processors, Computer Installation and Acceptance, and for the first time, Systems Management and Application Management embraces the Software Development Lifecycle, expanding the issues touched on in Software Lifecycle Support and Testing of IT Services.
Application Management - provides more detail on Business Change with emphasis being placed on clear requirement definition and implementation of solutions.
Business Perspective - fully understanding the nature of IT service provisioning and covers Business Continuity Management, Partnerships and Outsourcing. Surviving Change and Transformation of Business Practices through Radical Change.

There are many other publications in the library, however, these are considered to be the core.