The Open Source Definition
In 1998, Bruce Perens and Eric S. Raymond founded the Open Source Initiative (OSI) with the goal of promoting Open Source software in the business world. The term “Open Source” was proposed as an alternative to “Free Software” to avoid the ambiguity of the word “free” in English and reduce corporate resistance towards free software.
The OSI’s foundational document is the Open Source Definition (OSD), derived from the Debian Free Software Guidelines written by Perens himself. The OSD establishes ten criteria that a licence must meet to be certified as Open Source, including freedom of redistribution, access to source code, the ability to create derivative works, and no discrimination against persons, groups or fields of endeavour.
Raymond, author of the essay “The Cathedral and the Bazaar” (1997), provides the theoretical foundation of the movement, describing the distributed development model that characterises successful Open Source projects.
Copyleft licenses: GPL and LGPL
The GNU General Public License (GPL), created by Richard Stallman in 1989 and updated to version 2 in 1991, is the quintessential copyleft licence. The principle is straightforward: anyone who distributes software derived from GPL code must release it under the same licence, ensuring that freedoms propagate to every derivative work. This mechanism, called copyleft, prevents free code from being incorporated into proprietary products without returning modifications to the community.
The LGPL (Lesser GPL) applies the same principle but with a softer restriction: an LGPL library can be used (linked) by proprietary software without requiring the proprietary code to adopt the GPL. This makes it suitable for libraries intended to be available to closed commercial applications as well.
Permissive licenses: BSD, MIT, Apache
Permissive licences take a different approach: they allow anyone to use, modify and redistribute the code, including within proprietary products, with the sole obligation of retaining the copyright notice. The BSD licence (in its 2-clause and 3-clause variants) and the MIT licence are the most widespread examples. The Apache License 2.0, released by the Apache Software Foundation, adds an explicit patent grant, protecting users from patent claims by contributors.
How to choose a licence
Licence selection is a strategic decision. Copyleft (GPL) ensures that code remains free but may limit adoption in corporate contexts. Permissive licences (BSD, MIT) favour maximum diffusion but allow derivative code to be closed. The LGPL represents a compromise for libraries.
Licences are the legal foundation of the Open Source ecosystem: without a clear legal framework, code sharing would remain an act of trust without guarantees. The OSI, by certifying compliant licences, provides a point of reference for developers, companies and institutions.