ITIL reference documents are not open source and are only available by purchase.
ITIL is not a standard and does not provide normative definitions of terms. However, the ITIL version 3 glossary is representative of the ITIL position.
ITIL® V3 Glossary of Terms, Definitions and Acronyms best-management-practice.com
ITIL wiki wiki.en.it-processmaps.com