It refers to web server related software that belongs to open source category.

Apache is very well known for its key role in the World Wide Web’s growth in the initial phases. It recently crossed a milestone of having over 100 million web sites. They are commonly found on different operating systems including Windows, Unix and so on. Apache was one amongst the first web servers to provide an alternative to Netscape Corporation. It has however grown considerably since then. Apache has remained one of the most popular HTTP servers since 1996. According to a recent report, Apache has covered more than half of the websites on the internet. It is also one of the busiest servers known to the web.