There are 3 gradations:
OSS: means the source code is visible and open to see for everyone, it does not mean that the software has a GPL license, permitting the use and modification by everyone, nor that it is actually free software (as in free beer, gratis)
FOSS: means the same, but also free as in free beer, it can be used without paying license fees. This does not mean that the software is not license-encumbered, it may not have a GPL license. The problem with free but not libre open source licenses (e.g. the MIT license model, e.g. used by BSD, by Google Chromium and Android, by Canonical for their Mir display server and Unity DE, etc...) is twofold: the code can be stolen and turned into closed source software, for instance that's what BeOS/MacOS/OSX/iOS did with FreeBSD, thus creating a situation whereby a corporation can take advantage of years of development from open source developers without giving anything back to the community, or creating a situation whereby closed source commercial software is released for use on open source platforms, without inheriting any open source license model, which severally inhibits quality and innovation.
FLOSS: free and libre open source software is the real deal, it means that the code is gratis and has one of the GPL license models, meaning everyone can use it, improve it, extend it, etc... the main property of GPL licensed software is that all software that is derived from GPL licensed code, inherits that GPL license, and thus can't be closed. This is violated by companies all the time, e.g. the firmwares of IoT devices like TV sets or personal cloud-type storage devices (Drobo, WD, etc...), but technology catches up with those abuses all the time, either through obsolescence (because in order to maintain GPL licensed code you actually need the full force of the open source developer community, it can't be successfully maintained by any commercial outfit, not even IBM or Microsoft or Samsung are rich and powerful enough on their own to successfully maintain even a limited open source based project without turning to the world wide open source developer community), or through feature paralysis, because open source software just does not go obsolete as such as long as there is a single user left, and companies find it hard to sell new products in such circumstances without turning to programmed hardware obsolescence, which causes larger RMA numbers and is not economically viable.