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A release of a piece of software which is not merely a revision or a bug fix release but which contains substantial changes (e.g., an overhaul of the interface, change in compatibility).
Traditionally, major releases are numbered as X.0; for example, WordPerfect 6.0 is a major release, significantly different from any previous version; whereas WordPerfect 6.1 has only minor changes, and is, thus, only a revision.
See also major delivery.