What does the term “porcelain” mean in Git?

The term “porcelain” appears occasionally in the Git documentation. What does it mean?

“Porcelain” is the material from which toilets are usually made (and sometimes other fixtures such as washbasins). This is distinct from “plumbing” (the actual pipes and drains), where the porcelain provides a more user-friendly interface to the plumbing.

Git uses this terminology in analogy, to separate the low-level commands that users don’t usually need to use directly (the “plumbing”) from the more user-friendly high level commands (the “porcelain”).