The release cycle describes how long between new OS versions. Some like Fedora release a new numbered version every 6 months, others like Debian every two years.
About Release Cycles
A software release is a distribution of software code, documentation, and support materials. The software release life cycle is composed of discrete phases that describe the software's maturity as it advances from planning and development to release and support phases.
The term release candidate (RC) refers to a version with potential to be a final product, ready to release unless fatal bugs emerge. In this stage of product stabilization, all product features have been designed, coded and tested through one or more beta cycles with no known showstopper-class bug.
A release is called "code complete" when the development team agrees that no entirely new source code will be added to this release. There may still be source code changes to fix defects. There may still be changes to documentation and data files, and to the code for test cases or utilities. New code may be added in a future release.