Proprietary software is any software that is not free (as in freedom)/open source software. Such software denies users and creators their basic freedoms (freedom of unlimited use, studying, modifying and sharing) and is therefore evil; proprietary software is mostly capitalist software designed to abuse its user in some way. Proprietary code is often secret, not publicly accessible, but there are many programs whose source code is available but which is still proprietary because no one except the "owner" has any legal rights to fixing it, improving it or redistributing it.
Examples of proprietary software are MS Windows, MacOS, Adobe Photoshop and almost every game. Proprietary software is not only extremely harmful to culture, progress and society in general, it is downright dangerous and in some cases life-threatening; see for example cases of medical implants such as pacemakers running secret proprietary code whose creator and maintainer goes bankrupt and can no longer continue to maintain such devices already planted into bodies of people -- such cases have already appeared, see e.g. Autonomic Technologies nervous system implants.
Proprietary software licenses are usually called EULAs.
By extension besides proprietary software there also exist other proprietary works, for example proprietary art or databases -- these are all works that are not free cultural works. Even though for example a proprietary movie probably isn't IMMEDIATELY as dangerous as proprietary software, it may be just as dangerous to society in the long run.
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