What does the term "porcelain" mean in Git?

"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").

More importantly, the term "porcelain" applies to high-level commands, with output:

• meant to be readable by human
• not meant to be parsed
• susceptible to changes/evolutions

That is key: if you script, you should use if possible plumbing commands, with stable outputs. Not porcelain commands.

However, you can use the output of a porcelain command which has a --porcelain option in script (see below), like:

git status --porcelaingit push --porcelaingit blame --porcelain

Although git includes its own porcelain layer, its low-level commands are sufficient to support development of alternative porcelains.
The interface (input, output, set of options and the semantics) to these low-level commands are meant to be a lot more stable than Porcelain level commands, because these commands are primarily for scripted use.
The interface to Porcelain commands on the other hand are subject to change in order to improve the end user experience.

See "How do I programmatically determine if there are uncommitted changes?" as an example to using plumbing commands instead of porcelain ones.

Note: A porcelain command can have a --porcelain option.
For instance: git status --porcelain, which designates an output meant to be parsed.

--porcelain

Give the output in an easy-to-parse format for scripts. This is similar to the short output, but will remain stable across git versions and regardless of user configuration. See below for details.

The thread mentioned above details:

This is my fault, to some degree.
The "short-status" form is meant for human eyeballs, and was designed by Junio.
Some people wanted a scriptable status output, too, so I slapped a "--porcelain" on the same format that turns off configurable features like relative pathnames and colorizing, and makes an implicit promise that we won't make further changes to the format.
The idea was to prevent people from scripting around --short, because it was never intended to be stable.
So yeah, while --porcelain by itself is stable and scriptable, it is perhaps not the most friendly to parsers. The "-z --porcelain" format is much more so, and I would recommend it to anyone scripting around "git status"

That reflects the need, for git users, to using porcelain commands in their scripts!
But only with stable output (with --porcelain)

As commented by william-berg, the same goes for git push!

--porcelain

Produce machine-readable output.
The output status line for each ref will be tab-separated and sent to stdout instead of stderr.
The full symbolic names of the refs will be given.

As John Glassmyer proposes in the comments:

Perhaps the meaning of --porcelain here is "produce output suitable for consumption by porcelain scripts".

And that could be supported by the very first case of "--porcelain option" introduction
(before git status --porcelain, commit 6f15787, September 2009, git 1.7.0,
before git push --porcelain, commit 1965ff7, June 2009, git 1.6.4):

git blame --porcelain:

-p--porcelain

Show in a format designed for machine consumption.

Commit b5c698d, October 2006, git 1.4.4

The new option makes the command's native output format to emit output that is easier to handle by Porcelain.

The coinage and usage of the term "porcelain" in git was actually by Mike Taht, while otherwise losing a heated argument with Linus Torvalds.

http://www.gelato.unsw.edu.au/archives/git/0504/0881.html

In fact, one of my hopes was that other SCM's could just use the git plumbing.
But then I'd really suggest that you use "git" itself, not any "libgit". Ie you take all the plumbing as real programs, and instead of trying to link against individual routines, you'd script it.

If you don't want it, I won't do it.
Still makes sense to separate the plumbing from the porcelain, though.