From use, it seems like "Source Available" generally means "it's not open source, but you can go and read the code somewhere." So it's more a definition by contrast, really. "Fair Source" is a new term defined more precisely by https://fair.io/about/ - it seems to refer to be a more specific term referring to this specific license type.

Exactly. The problem with "Source Available" is that it has no real definition and it conveys no user freedoms outside of the (loose) freedom to read the source code, which is why the term has never taken off with anybody. It's wholly inadequate. That's why we needed a new term that does have a concrete definition and does convey user freedoms. Fair Source is meant to sit on the gradient between Open Source and Source Available.

It seems that the only relevant license in context is the Functional Source License (FSL).

What's the point of this new term only encompassing the accompanying new license from the same authors, other than to manipulate our minds to associate "FSL" with "fair"?

FSL is a source-available license. "Fair Source" is the name of an industry lobby group. Uncritically adopting their retaxonomization of software licenses and newspeak is not advisable.

The FSL seems like a valuable contribution and can make sense for companies like GitButler and Sentry. The accompanying lobbying and propaganda (aka "PR") spoils that somewhat.

Not just FSL. There's also the Fair Core License [0], the Business Source License [1], and we're open to adding more given they abide by the Fair Source Definition. We're just getting started.

[0]: https://fcl.dev

[1]: https://mariadb.com/bsl-faq-mariadb/