A pure function is one which does not perform any side effectsᛦ.
Nix, to quote the marketing copy, is a purely functional package manager. That is, a package is an immutable value constructed by a pure functionᛦ. The value proposition is that packages comprise a closure over a set of dependencies, such that packages are guaranteed to build the same way every time without drift. This also allows for multiple versions of packages (and most notably their dependencies) to coexist on the same system without conflict.