title: What is the "active Nix expression"?
body: <p>I notice that descriptions of various nixos commands refer to something called the "active Nix expression". For example, the man page for <code>nix-env</code> includes:</p>
  
  <pre><code>   --file / -f path
         Specifies the Nix expression (designated below as the active Nix expression) used by the --install, --upgrade, and --query
         --available operations to obtain derivations. The default is ~/.nix-defexpr.
  </code></pre>
  
  <p>What is this "active Nix expression"? Where is it defined? Is it simply what is written in <code>/etc/nixos/configuration.nix</code> ordinarily or instead what was defined by <code>nix-shell</code> otherwise?</p>