This website attempts to define just that. Open source does not only mean that the source code is available for a developer community. To be considered truly open source, software has to fulfill the following licensing, sharing, and legal conditions:
1. Free Redistribution 
2. Source Code
3. Derived Works
4. Integrity of The Author’s Source Code
5. No Discrimination Against Persons or Groups
6. No Discrimination Against Fields of Endeavor
7. Distribution of License
8. License Must Not Be Specific to a Product
9. License Must Not Restrict Other Software
10. License Must Be Technology-Neutral