Nginx is an open source, high-performance web server that goes far beyond its traditional role. It also functions as a reverse proxy, load balancer, and HTTP cache, making it a cornerstone of modern web infrastructure. Its popularity is due to its unique, event-driven, and asynchronous architecture, which allows it to handle tens of thousands of simultaneous connections with minimal resource consumption. The software was developed by Igor Sysoev to solve the C10k problem, and has received key improvements over its history, such as support for HTTP/2 and dynamic modules. This encyclopedia entry provides a detailed introduction to Nginx's master-worker process model, hierarchical configuration system, and key uses. Finally, we compare it with Apache HTTP Server, highlighting key differences in performance, configuration, and flexibility.