nginx (pronounced “Engine-X”) is an open source Web server and a reverse proxy server for HTTP, SMTP, POP3 and IMAP protocols, with a strong focus on high concurrency, performance and low memory usage. It is licensed under a BSD-like license and it runs on Unix, Linux, BSD variants, Mac OS X, Solaris, AIX and Microsoft Windows.

Nginx can deploy dynamic HTTP content on a network using FastCGI, SCGI handlers for scripts, uWSGI application servers or Phusion Passenger module, and can serve as a software load balancer.

Nginx uses an asynchronous event-driven approach to handling requests which can provide more predictable performance under high loads, in contrast to the Apache HTTP server model that defaults to a threaded or process-oriented approach to handling requests.

Originally, nginx was developed to fill the needs of various websites run by Rambler, for which it was serving 500 million requests per day as of September 2008.

According to Netcraft's March 2012 Web Server Survey, nginx was found to be the third most widely used web server across all domains (10.15% of surveyed sites) and the second most widely used web server for all "active" sites (12.77% of surveyed sites). According to W3Techs it is used by 10.6% of the top 1 million websites, and by 25.6% of the top 1,000 websites. According to BuiltWith, it is used on 13.76% of the top 10,000 websites, and its growth within the top 10k, 100k and 1 million segments is increasing.