What is the difference between git fetch and git fetch origin?

What is the difference between these two commands?

git fetch

and

git fetch origin

This is simpler than the linked answer (which is really about what name(s) are updated with git fetch origin vs git fetch origin master vs git fetch origin master:master; and that answer is slightly different for very old versions of Git as its behavior changed a bit in Git version 1.8.4).

To quote the git fetch documentation:

When no remote is specified, by default the origin remote will be used, unless there’s an upstream branch configured for the current branch.

In other words:

git fetch

is exactly the same as:

git fetch origin

unless the current branch has an upstream setting that refers to a remote other than origin. So if, for instance, the current branch is rumpelsnakeskin and the upstream of rumpelsnakeskin is habitat/rumpelsnakeskin, then git fetch means git fetch habitat. But if the current branch is, say, master and its upstream is origin/master or is not set at all, then git fetch means git fetch origin.