Proprietary operating system is one which a particular company conceptualizes, designs, develops and sells. Examples of proprietary operating systems are Windows and Mac OS X. These operating systems are designed and sold by these companies and are not meant to be tampered with or tweaked by users.
A proprietary system may be frustrating for individuals who like to control and tweak various aspects of their operating system. Companies who design proprietary operating systems may make it intentionally difficult, and sometimes impossible, to change or even view the source code. Software companies generally want to protect the integrity of their product and prevent individuals from stealing proprietary knowledge by viewing sensitive source code.