The GNU Lesser General Public License (LGPL) is a free-software license published by the Free Software Foundation (FSF). The license allows developers and companies to use and integrate a software component released under the LGPL into their own (even proprietary) software without being required by the terms of a strong copyleft license to release the source code of their own components. However, any developer who modifies an LGPL-covered component is required to make their modified version available under the same LGPL license. For proprietary software, code under the LGPL is usually used in the form of a shared library, so that there is a clear separation between the proprietary and LGPL components.
The main difference between the GPL and the LGPL is that the latter allows the work to be linked with (in the case of a library, «used by») a non-(L)GPLed program, regardless of whether it is free software or proprietary software. The non-(L)GPLed program can then be distributed under any terms if it is not a derivative work. If it is a derivative work, then the program’s terms must allow for «modification for the customer’s own use and reverse engineering for debugging such modifications.» Whether a work that uses an LGPL program is a derivative work or not is a legal issue.