Hermeticity is a property of Nix builds, which isolates them from the host system via various mechanisms. This results in a system where the same set of source inputs will always map to the same build outputs, because changes on the host can not affect a build.
Any system which is not hermetic is called "impure", implying that changes outside of the input set can have an impact on the build result. This is the inverse of Nix's "purity" property.