Just a note on nomenclature: I recognize that some people have very specific recommendations about what language to use when talking about software freedom. I appreciate the analysis behind these recommendations and largely agree with them (despite my other concerns about these folk edit dja 20211113).
Even so, I find using just the word "Linux" alone to talk about operating systems which use the Linux kernel to be a reasonable pars pro toto figure of speech.
Further, I like the phrase free and open source software or just FOSS fine for talking about software available to run, study, modify, and distribute under FSF-approved licenses, for a number of reasons.
To start, the ambiguity of the English word free is not going away, and despite literally decades talking about it--clarifying we mean freedom and not price, and about how the Romance languages with their libre vs gratis pairing have it better--it remains a source of confusion and, more detrimentally, a source of frustration.
When the two terms are combined, the idea that using the two-word, eleven-character phrase open source privileges that term over free because it is longer, or because of where it falls in the ordering of the combination, can only make sense to someone unable to grasp the considerable appeal of concision. All things being equal, shorter is better. The only reason the much shorter word free doesn't suffice here is because of its own inherent limitations, mentioned above, and has nothing to do with the relative merits of the phrase open source.
On the contrary, pairing free with open source both gets across the idea that open source is not alone sufficient, and at least hints at the kind of free we mean. The combination can be more than the sum of its parts.
At which point, the addition of the affectatious loanword libre adds little but length, and turns a moderately distinct and searchable acronym FOSS into a horribly overloaded (for our purposes) dictionary word FLOSS.1
In speaking, I tend to switch it up amongst using free software, open source, and FOSS perhaps depending on who I'm talking to, where, about what. If I'm talking to people I trust to understand which version of free I mean, I'll use free software. If I want a single syllable or acronym, FOSS it is.
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FOSS is not perfect as a search term, since it is used as an acronym for other phrases, and is a personal surname, but it is orders of magnitude better than floss. ↩