Definition: (Webopedia)
Open source doesn't just mean access to the source code. The distribution terms of open-source software must comply with the following criteria: (Full version available here)

• Free Redistribution
• must include Source Code
• Derived Works
• Integrity of The Author's Source Code
• No Discrimination Against Persons or Groups
• No Discrimination Against Fields of Endeavor
• Distribution of License
• License Must Not Be Specific to a Product
• License Must Not Restrict Other Software
• License Must Be Technology-Neutral