Nginx reverse proxy, two applications with different domain names on the same server?

Server names in nginx are defined using the server_name directive and determine which server block is used for a given request.

For example:

server {
    listen       80;
    server_name  example.org  www.example.org;
    ...
}

server {
    listen       80;
    server_name  server.org;
    ...
}

There are some server names that are treated specially.

Catch-all server uses the name “_”:

server {
        listen       80  default_server;
        server_name  _;
    }

More details can be found here: Nginx Server names