Gecko is a free and open source layout engine used in many applications developed by Mozilla Foundation and the Mozilla Corporation (notably Firefox web browser), as well as in many other open source software projects.
It is designed to support open Internet standards, and is used by different applications to display web pages and, in some cases, an application's user interface itself (by rendering XUL). Gecko offers a rich programming API that makes it suitable for a wide variety of roles in Internet-enabled applications, such as web browsers, content presentation, and client/server.
Gecko is written in C++ and is cross-platform, and runs on various operating systems including BSDs, Linux, Mac OS X, Solaris, OS/2, AIX, OpenVMS, and Microsoft Windows. Its development is now overseen by the Mozilla Foundation and is licensed by a tri-license of the Mozilla Public License (MPL), GNU General Public License (GPL) and GNU Lesser General Public License (LGPL).
Gecko is the second most-popular layout engine on the World Wide Web, after Trident (used by Internet Explorer for Windows since version 4), and followed by WebKit (used by Safari & Google Chrome) and Presto (used by Opera).