A ChangeLog is a file
used by the author
(s) and/or maintainer
(s) of a program
to record change
s made to the program's implementation
This contrasts with the NEWS file, which is used to document only user-visible changes to the program.
The GNU Coding Standards recommend that anyone who modifies and distributes as a new version a piece of free software record their changes clearly in the ChangeLog, so that problems caused by the changes will not be referred to the official maintainer.
ChangeLog is the canonical spelling.