Definition
NGINX (pronounced "engine-x") is a high-performance open-source web server and reverse proxy that powers a large share of the modern web. It serves static files directly, proxies traffic to application backends such as PHP-FPM, Node.js and Python WSGI/ASGI servers, terminates SSL/TLS, load-balances across multiple upstream servers and caches HTTP responses to cut backend load. NGINX is the default web server on most VPS and dedicated Linux hosts, having overtaken Apache as the standard for high-traffic sites because of its asynchronous, event-driven architecture: a small, fixed number of worker processes handle thousands of connections each through non-blocking I/O, rather than Apache's traditional model of one process or thread per connection, which uses far more memory under load. This matters most above a few hundred concurrent users; for a small brochure site the choice barely registers. Most managed WordPress hosts run NGINX, often paired with FastCGI cache, under the hood, though .htaccess-based configuration does not carry over.
How it works
NGINX is configured through a hierarchical text file (nginx.conf) that defines servers, locations, upstreams and caches. It can be a pure web server, a pure reverse proxy, or both. NGINX Plus is the paid commercial version with extra features; the open-source NGINX is what most sites actually run.
Why it matters
For static content and reverse proxying, NGINX is faster and lighter than Apache. For high-traffic sites, the difference matters: NGINX can handle tens of thousands of concurrent connections on modest hardware. Most production stacks now use NGINX in front of an application server, with Apache reserved for legacy compatibility.