Package File (OS X)
A Package File is used in Mac OS X as an installation archive of a software package for distribution. Clicking on it on an OS X system will launch the Installer package to extract and install the software.
These files have a .pkg extension, an extension that has also been used for installable packages on a number of other operating systems including SunOS, Unix System V, Symbian, BeOS, and the Apple Newton.
OS X packages are a form of the xar archive format.
A Meta-Package File is another form of OS X package which may contain multiple packages of the single-package format.