related to F/LOS, F/LOSS, etc.

Open source is a flavor of Free/Libre stuff. Open source specifically refers to a more corporate friendly version of Free Software that Eric Raymond and some others thought would be an easier sell than GNU's GPL version! The MIT License is the most notable variation of open source licensing.

Open source these days sort of refers to ANYTHING that you're freely sharing and hoping other people just use for whatever they want. It also frequently is used incorrectly to refer to public domain things, or free, but not as in freedom things (incorrectly used re: GRATIS not LIBRE things).