Proprietary software is software whose owner withholds the source code and restricts how the program may be used, copied, modified, or redistributed. The GNU project’s “Categories of free and nonfree software” page states the definition bluntly: “Proprietary software is another name for nonfree software,” and it defines nonfree software as “any software that is not free,” whose “use, redistribution or modification is prohibited, or requires you to ask for permission, or is restricted so much that you effectively can’t do it freely.” In this framing the defining feature is not secrecy of source per se but the absence of the user’s freedom to control the program; withheld source is the usual means by which that control is denied.
The concept is best understood as the negative space against which free software defined itself. The Free Software Definition lists four essential freedoms - to run the program for any purpose, to study and change it (which requires access to source code), to redistribute copies, and to distribute modified versions. A program that denies any of these is, by that definition, nonfree, and proprietary software is the broad category of programs that deny several at once. Where free software treats the user as someone entitled to inspect and reshape their own tools, proprietary software treats the program as a product the vendor licenses out under terms it controls.
The legal instrument that enforces those terms is the end-user license agreement, or EULA. Rather than selling a copy of the software outright, a proprietary vendor typically licenses its use subject to conditions: no reverse engineering, no redistribution, no running on more machines than purchased, no modification, and often disclaimers of warranty and limits on liability. The combination of a withheld source tree and a restrictive license is what operationally produces proprietary software. The user receives a binary they may run only within the boundaries the vendor has drawn, and lacks both the technical means (no source) and the legal permission (the EULA) to step outside them.
This category was the entire motivation for the free software movement. Richard Stallman has repeatedly described the experience of being unable to fix a printer driver because its source was secret as the spark for founding the GNU project in 1983. The movement’s vocabulary - free software, copyleft, the GNU General Public License - exists to construct a deliberate alternative to the proprietary model, in which freedoms are guaranteed rather than withheld. The later open source movement shared the same technical practice of sharing source while framing its appeal around engineering quality rather than user liberty, but both defined themselves in opposition to the closed, EULA-governed proprietary norm.
Proprietary software remains the dominant commercial model for much of the industry, and the GNU project’s categories page is careful to note that nonfree is a spectrum: some proprietary programs are merely closed, while others actively restrict or surveil their users. The free software community uses “proprietary” not as a neutral description of a business model but as a value judgment about who holds power over the software - the vendor or the user. Understanding that the term carries this charge is essential to reading the licensing debates of the 1980s through the 2000s, in which nearly every other concept in this part of the history was articulated as a response to proprietary control.