Short definition
nginx.conf is the main configuration file for nginx, defining global server behaviour including worker processes, connection handling, logging, compression, and the HTTP context that all virtual hosts inherit from.
Extended definition
Every nginx instance reads nginx.conf at startup. It is the root of the entire configuration tree. The file is structured into contexts: the top-level main context, an events context for connection handling, and an http context for all web traffic behaviour. Virtual host files (server blocks) are typically included into the http context from separate files, keeping nginx.conf clean and focused on global settings. Changes to nginx.conf require either a reload or restart of nginx to take effect, and should always be validated with nginx -t before applying.
Deep technical explanation
Context hierarchy: nginx.conf is organised into nested contexts. Directives in an outer context are inherited by inner contexts unless overridden. The main context contains global settings. The events context controls I/O handling. The http context contains all HTTP-related settings and includes server blocks.
Worker processes and connections: worker_processes auto instructs nginx to spawn one worker per CPU core. worker_connections sets the maximum simultaneous connections per worker. Total capacity is worker processes multiplied by worker connections.
Include directives: The include directive pulls in external files. The standard Ubuntu nginx setup uses include /etc/nginx/sites-enabled/* inside the http block to load all active vhost configurations. This keeps nginx.conf minimal.
Gzip and caching: Global compression settings, proxy cache paths, and buffer sizes are defined in the http context of nginx.conf, making them available to all server blocks without repetition.
Error and access logging: Default log file paths are defined in nginx.conf. Individual server blocks can override these with their own log directives, but the global defaults apply to anything not explicitly configured.
Why it matters
nginx.conf is the foundation that everything else inherits from. A poorly configured nginx.conf affects every site and application on the server. Understanding its structure is a prerequisite for safely managing any nginx deployment at scale.