What Is Open Source Software?

The Full Definition

At its simplest, open source software is defined by access to source code, the human-readable instructions that tell a computer what to do. When developers write software, they work with source code written in programming languages like Python, C, Java, or JavaScript. This source code is then compiled or interpreted into machine-readable instructions that computers execute. With proprietary software, users receive only the compiled version and have no way to see, understand, or change the underlying code. Open source reverses this by making the source code available to everyone.

The Open Source Initiative, the organization that stewards the formal Open Source Definition, specifies ten criteria that a software license must meet to qualify as open source. These include free redistribution, access to source code, permission to create derived works, no discrimination against persons or fields of endeavor, and technology neutrality. Software that merely publishes its source code without granting these freedoms is considered "source-available" rather than truly open source.

The distinction matters because open source is not just about visibility. It is about rights. A project that lets you read the code but prohibits you from modifying it, or one that allows modifications but restricts commercial use, does not meet the open source definition. The legal protections encoded in open source licenses are what make the entire ecosystem of shared development possible.

How It Differs from Other Software Models

Proprietary software, sometimes called closed-source software, is owned and controlled by the company or individual that created it. The source code is a trade secret, and users agree to license terms that typically prohibit reverse engineering, modification, and redistribution. Examples include Microsoft Windows, Adobe Photoshop, and most commercial mobile apps.

Freeware is software distributed at no cost but without source code access. Users can run the program but cannot study, modify, or redistribute it. Many mobile apps and browser extensions fall into this category. Freeware should not be confused with free software or open source, even though all three may be available without payment.

Source-available software makes its code visible but attaches restrictions that prevent it from qualifying as open source. Some companies release source code under licenses that prohibit commercial use, restrict certain deployment models, or require special agreements for enterprise use. These projects may look like open source on the surface, but the license terms limit the freedoms that define genuine open source.

Free software, as defined by the Free Software Foundation, is closely related to open source but emphasizes philosophical freedom rather than practical development methodology. The Free Software Foundation articulates four essential freedoms: the freedom to run the program for any purpose, to study how it works and change it, to redistribute copies, and to distribute modified versions. In practice, most free software also qualifies as open source, and most open source software qualifies as free software. The differences are primarily philosophical and political rather than technical.

Why Open Source Matters

Open source software matters because it democratizes access to technology. When source code is freely available, individuals and organizations in every country can use, study, and build upon the same tools, regardless of their budget. This is particularly significant in education, where students can examine real production code to learn how software works, and in developing economies, where licensing costs for proprietary software can be prohibitive.

The transparency of open source also serves as a check on power. When governments deploy open source software for elections, public services, or surveillance, citizens and independent auditors can verify what the code actually does. This accountability is impossible with proprietary software, where users must trust the vendor's assurances without the ability to verify them independently.

From an innovation standpoint, open source accelerates progress by allowing developers to build on existing work rather than starting from scratch. The machine learning revolution, for instance, has been powered largely by open source frameworks like TensorFlow and PyTorch. Without the open sharing of these tools, the rapid advances in artificial intelligence would have been far slower and concentrated among a handful of well-funded companies.

Examples You Already Use

Most people interact with open source software daily without realizing it. Android, the operating system running on billions of smartphones, is built on the Linux kernel and released under open source licenses. The Chrome browser is based on the open source Chromium project. Firefox is entirely open source. WordPress powers more than 40 percent of all websites, meaning that a significant portion of the content people read online is served by open source software.

Behind the scenes, the infrastructure of the internet itself depends on open source. DNS servers, web servers, email servers, and networking tools are predominantly open source. When you send a message, stream a video, or make an online purchase, your data passes through multiple layers of open source software before reaching its destination.