What is free software? – GNU Project – Free Software Foundation
The Free Software Definition
The free software definition presents the criteria for whether a particular software program qualifies as free software. From time to time we revise this definition, to clarify it or to resolve questions about subtle issues. See the History section below for a list of changes that affect the definition of free software.
Free software means software that respects users' freedom and community. Roughly, the users have the freedom to run, copy, distribute, study, change and improve the software.
Thus, free software is a matter of liberty, not price. To understand the concept, you should think of free as in free speech, not as in free beer.
With these freedoms, the users (both individually and collectively) control the program and what it does for them. When users don't control the program, the program controls the users. The developer controls the program, and through it exercises power over the users. Therefore, a nonfree or proprietary program is an instrument of unjust power.
A program is free software if the program's users have the four essential freedoms:
A program is free software if it gives users adequately all of these freedoms. Otherwise, it is nonfree. While we can distinguish various nonfree distribution schemes in terms of how far they fall short of being free, we consider them all equally unethical.
The rest of this page clarifies certain points about what makes specific freedoms adequate or not.
Freedom to distribute (freedoms 2 and 3) means you are free to redistribute copies, either with or without modifications, either gratis or charging a fee for distribution, to anyone anywhere. Being free to do these things means (among other things) that you do not have to ask or pay for permission to do so.
You should also have the freedom to make modifications and use them privately in your own work or play, without even mentioning that they exist. If you do publish your changes, you should not be required to notify anyone in particular, or in any particular way.
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What is free software? - GNU Project - Free Software Foundation