GLOSSARY
What is LGPL?
LGPL is the GNU Lesser General Public License, a weaker copyleft license designed so a library can be linked into proprietary software under conditions. This glossary entry defines it plainly for enterprises and explains where the obligation still bites.
Commercial and licensing risk advisory, not legal advice.
Definition
LGPL stands for the GNU Lesser General Public License, a copyleft open source license from the Free Software Foundation and OSI approved open source. It is described as lesser because it applies a weaker form of copyleft than the GPL. The point of the LGPL is to let a library be used by proprietary software without forcing that software to adopt the full copyleft. You can link an LGPL library into a closed application, provided you meet conditions that preserve the user's ability to replace the library with a modified version. Changes you make to the LGPL library itself remain under the license, but the application that merely links to it does not inherit copyleft the way it would under the GPL.
How the linking allowance works
The mechanism that makes the LGPL workable for proprietary products is the linking allowance. The license generally requires that users be able to substitute their own modified version of the LGPL component and relink it into the program. In practice this favors dynamic linking, where the library is loaded separately and can be swapped, or providing the object files needed to relink a statically linked build. Alongside that, you share the source of the LGPL library and any modifications you made to it. Static linking is permitted, but it raises the relinking obligation more sharply, which is why how you link an LGPL component is a question worth answering precisely rather than assuming.
This is the practical heart of the LGPL. The obligation is not heavy for most uses, but it is conditional, and the conditions depend on engineering details such as linking style. Those details are exactly what a dependency review should capture.
Why it matters to enterprises
The LGPL is common in proprietary products because of its linking allowance, which makes it tempting to treat as obligation free. It is not. The relinking and source conditions still apply, and meeting them depends on how the library is incorporated. For an enterprise, the task is to identify LGPL components, record how each is linked, and confirm that the conditions are satisfied, leaving interpretation to counsel. The linking style question is closely tied to the concept of a linking exception, which we cover in the glossary entry on what is a linking exception. For a stronger copyleft term and its network trigger, see what is GNU AGPL, and browse the full open source license risk glossary for the surrounding vocabulary.
COMMON QUESTIONS
Questions buyers ask.
What is LGPL?
LGPL is the GNU Lesser General Public License, a weaker copyleft open source license from the Free Software Foundation. It lets you link an LGPL library into proprietary software without that software inheriting the full copyleft, provided you meet conditions that let users replace the library. It is OSI approved open source.
How is LGPL different from GPL?
The GPL applies strong copyleft to the whole combined work, so linking GPL code into your application can subject the application to the GPL. The LGPL relaxes this for linking, allowing proprietary software to use an LGPL library if users can substitute their own version of that library. Modifications to the library itself remain copyleft.
What conditions does the LGPL place on linking?
The LGPL generally requires that users be able to replace the LGPL component with a modified version and relink. In practice this favors dynamic linking or providing object files for relinking, along with sharing source of the LGPL library and any modifications. Static linking is permitted but raises the relinking obligation, which your own counsel should review.
Is LGPL safe for proprietary products?
LGPL is widely used in proprietary products precisely because of its linking allowance, but it is not obligation free. The relinking and source conditions must be met, and how you link the library matters. Identifying LGPL components and how they are linked is the practical task, with interpretation left to your own counsel.
FIND YOUR COPYLEFT EXPOSURE
Know how your LGPL components are linked.
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