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The Apache HTTP Server, commonly referred to as Apache, is a web server notable for playing a key role in the initial growth of the World Wide Web and in 2009 became the first web server to surpass the 100 million web site milestone. Apache was the first viable alternative to the Netscape Communications Corporation web server (currently known as Sun Java System Web Server), and has since evolved to rival other Unix-based web servers in terms of functionality and performance. The majority of all web servers using Apache are Linux web servers.

Apache is developed and maintained by an open community of developers under the auspices of the Apache Software Foundation. The application is available for a wide variety of operating systems, including Unix, GNU, FreeBSD, Linux, Solaris, Novell NetWare, Mac OS X, Microsoft Windows, OS/2, TPF, and eComStation. Released under the Apache License, Apache is characterized as free software and open source software.

Since April 1996 Apache has been the most popular HTTP server on the World Wide Web. As of March 2009[update] Apache served over 46% of all websites and over 66% of the million busiest.