Dictionaries are descriptive, not prescriptive. The meaning of terms outside of specific fields like law is based on consensus, not authority. Merriam-Webster doesn't own the English language, and neither does OSI.
If you want to look for consensus, there's no better place than Wikipedia.
> Open-source software (OSS) is computer software that is released under a license in which the copyright holder grants users the rights to use, study, change, and distribute the software and its source code to anyone and for any purpose.
