Clarify the definition of "derivation"

[fricklerhandwerk]

Problem

At least five users find the concept of "derivation" as described in the manual highly confusing:

@Soupstraw @ianthehenry @apoelstra myself and others.

Related:

• https://ianthehenry.com/posts/how-to-learn-nix/glossary/#derivation
• https://discourse.nixos.org/t/usability-study-session-2/21400

Proposal

Reword it such at it both reflects how Nix currently works and also explains the concept such that people can develop an appropriate mental model regardless of how it's actually implemented.

Checklist

• checked latest Nix manual (source)
• checked open documentation issues and pull requests for possible duplicates

Priorities

Add 👍 to issues you find important.

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/tweag-nix-dev-update-44/25546/1

[kephas]

I'll note that even in the Reference Manual, in which I would expect full definitions and technical details, neither 5.4 Derivations nor 7.5.8. nix derivation show fully explain what a derivation is and contains (in the .drv file).

As a developer, it is indeed confusing not to find the actual details anywhere, because that's often a way to wrap our head around concepts.