alt Hacker NewsFree Software and Open Source have definitions that are both on their face and in practical application by the bodies responsible for each almost entirely identical. Neither is a subset of the other.
If you are concerned about mandating users provide modifications by a similar license to the one they received material under, what you want is copyleft.
“Free software” means copyleft. The free software foundation manages copyleft licenses. The term open source was explicitly coined to differentiate from the more restrictive free software / copyleft.
All free software licenses are open source licenses. Not all open source licenses are free software licenses.