The GNU Lesser General Public License is the GNU Project’s weaker-copyleft license, intended primarily for software libraries. It occupies a middle ground between the strong reciprocity of the GPL and the near-total permissiveness of licenses like the MIT or BSD. Its central effect is to allow a program that merely links to an LGPL-covered library to be distributed under terms of the distributor’s choosing, including proprietary terms, while the library itself remains free.
Version 3 of the LGPL, published on 29 June 2007 alongside GPLv3, is constructed in an unusually economical way. Rather than restating an entire license, it is written as a short set of additional permissions layered on top of GPLv3. The text opens: “This version of the GNU Lesser General Public License incorporates the terms and conditions of version 3 of the GNU General Public License, supplemented by the additional permissions listed below.” This means the LGPL inherits all of the GPL’s machinery and then relaxes specific obligations for the case of a library combined with other code.
The key relaxation concerns what the LGPL calls a “Combined Work”: a work produced by linking or combining an “Application” with the covered “Library.” A distributor may convey such a Combined Work under terms of their choice, provided they meet certain conditions that preserve the user’s ability to replace or modify the LGPL-covered portion. In practice this lets the library be used by proprietary programs, while any changes to the library’s own source must still be released under the LGPL.
This boundary is deliberate. Modifications and extensions to the library are copyleft, so improvements to the shared component flow back to the community, but a program that simply uses the library through its published interface is not pulled under copyleft. That trade-off is why the LGPL is sometimes chosen for foundational libraries whose authors want broad adoption, including by non-free software, without surrendering the freedom of the library code itself.
The Free Software Foundation has historically been cautious about recommending the LGPL for every library, arguing in its philosophy writings that strong copyleft is often the more strategic choice and that “Lesser” GPL terms should be used deliberately rather than by default. The LGPL nonetheless remains one of the most widely deployed free software licenses, precisely because it reconciles a free library with the realities of a mixed software ecosystem.