Acronym for Concurrent Versions System, a VersionControlSystem. (Not to be confused with CSV.)
It was born as a batch of Shell scripts wrapping RCS and diff3(1), but evolved into something much greater. Unfortunately, its roots also imply some annoying limitations like being unable to rename/move files in the repository without losing the associated history.
See also:
Open Source Development with CVS by Coriolis Open Press.
Note that some (irrelevant?) chapters regarding the philosophy behind CVS in OpenSource are only present in the printed edition.
30 pages link to CVS: