Not at all to defend Meta but "a feature that allows software developers to test applications" is a dubious definition of localhost. I also can't come up with a better one.

It's not a definition, but it is an accurate statement.

It's a true statement but I'm not sure it gives a good impression of what localhost actually is.

“A loopback network interface” or “A interface that refers to the same host”.

“A network interface which allows processes on the same internet host to communicate without the need for a network connection”

There's a lot of layperson-unfriendly words in there! Iterating on that:

"A feature that allows multiple programs on the same device to communicate without the need for an internet connection"

Some concepts just can't (or shouldn't) be broken down to the level of lay person friendly though. There are just some technical concepts that have a complexity floor that if you drop below you are no longer explaining the actual concept but a fantasy.

For a judge trying to rule on a technical case, a poor layperson analogy and lead to a confidently wrong legal conclusion that has serious negative consequences. Thats why court appointed neutral experts are important.

A way for computer programs to talk to each other on the same device as though they were running on different devices connected over a network.

I agree with you by the way, I just don’t think this is one of those cases.

Computer talky no pluggy

a pty fits that definition though

It is like having a pool room at home instead of playing at the bar. Facebook want to snoop around your pool room.