FOSS is free and open source software, focused on the freedom to do whatever you want with the source code and redistribute it (typically mentioning the original project and etc.)
Open source software only indicates that the software is transparant and its source code is public.
I tought people just used different ways to write open source. Some with a dash, others not. I use them both, depends on my laziness level... Have you encountered the answer yet? I am quite curious...