Programming paradigm based on applying and composing functions| en.wikipedia.org
Overlays are Nix functions which accept two arguments, conventionally called final and prev (formerly also self and super), and return a set of packages. ... Overlays are similar to other methods for customizing Nixpkgs, in particular the packageOverrides ... Indeed, packageOverrides acts as an overlay with only the prev (super) argument. It is therefore appropriate for basic use, but overlays are more powerful and easier to distribute.| nixos.wiki