Free Software (FSF) vs. Open Source (OSI)
Most free/libre software is both, Free Software (as defined by the FSF) and Open Source (as defined by the OSI). But there are exceptions, so not all Free Software is Open Source, not all Open Source is Free Software.
If we want to care for these exceptions, we’d need one tag for FSF’s definition (for example: free-software) and one tag for OSI’s definition (for example: open-source). Personally, I’d like to have that, but I guess this would be too elaborate for this site. So for now, let’s concentrate on the common case that a software falls under both definitions.
There are various philosophical/political/ideological disputes about which term to use to refer to such software. That’s why there are alternative terms. Among them:
- The term FOSS, which stands for Free and Open Source Software.
- The term FLOSS, which stands for Free/Libre/Open Source Software.
Because FLOSS refers to "libre" (= freedom in many languages, without additional meaning of "free" (as in gratis) in English), FLOSS should be preferred to FOSS.
I think we should use floss for software that must be free/libre.
Advantages of using floss:
- We would be neutral. We don’t take part in the disputes by deciding to use one of the two competing terms.
- If we use only one of the competing terms instead, we’d explicitly exclude the other definition. Then what to use for software that only falls under the not-used-as-tag definition? Using FLOSS makes clear that both definitions apply.
- If we’d use open-source, some users might not be aware of OSI’s definition and think that it refers to any software that got its source code published.
- If we’d use free-software, some users might not be aware of FSF’s definition and think it refers to any software that is available for free/gratis.
Disadvantages of using floss:
- The term is not very popular.
But I don’t think this is a problem. It is in use for years. Richard Stallman recommends it when you don’t want to take sides. Wikipedia mentions that it was used by the European Commission, South Africa, Spain, and Brazil.
Possible synonyms for floss: free-software, open-source, open-source-software, foss, libre ….
Software available for free
If software must be free as in beer, we could use gratis. free is too ambiguous.
Possible synonyms for gratis: free, freeware, ….
FLOSS may cost money
Note that both tags might be used on the same question, as FLOSS doesn’t need to be available gratis.
However, it is gratis in most cases. I’d only use it in the following scenario:
"You look for gratis FLOSS recommendations and you already know that there is a FLOSS solution which costs money."
In general it should be sufficient to use floss without gratis. But answers recommending FLOSS for sale would still be on-topic.
Only use these tags for requirements
If one doesn’t care if the software is proprietary or FLOSS, as long as it is gratis, the tag floss should not be used.
See the question: "Feature tags" only for required or also for optional features?
There is no "official" definition
There is no institute or something like that which defines the scope of "FLOSS". So while it seems likely that it means "Software that is both, Open Source and Free Software", it could also be understood as "Software that is Open Source and/or Free Software" (I don’t endorse it). That could be a "hack" to solve the mentioned problem of exceptions (where a software only applies to one, not both definitions).