Open Source
Etymology
The phrase was coined in 1998 by Christine Peterson, proposed as a more business-friendly banner than the older "free software" after Netscape released its browser’s source code. "Source" here means source code — the human-readable instructions before they’re compiled. The movement reframed openness as a practical advantage in reliability and trust, not only an ethical stance.
noun phrase · /OH-pun SORSS/
Software whose underlying code is published for anyone to read, use, modify, and share.
What it names
A whole model of building things in the open — the reason much of the internet runs on software no single company owns.
Example
They chose an open-source database so they could inspect the code and never get locked in.
Related idea
Open source is about the code being visible and modifiable, which is not the same as free in price: much open-source software is free, but the defining freedom is the right to see and change it.