Open source software is defined by its attached license guaranteeing anybody rights to freely use, modify, and redistribute the software.

Open Source is a certification mark owned by the Open Source Initiative. Developers of software that is intended to be freely shared and possibly improved and redistributed by others can use the Open Source trademark if their distribution terms conform to the OSI's Open Source Definition.

To summarize, the Definition model of distribution terms require that:

• Free Redistribution
  No restrictions are placed on parties from selling of giving away the software.
• Source Code Availability
  The software must include source code and must also allow for binary distributions when there is a well-publicized means of obtaining the source code.
• Derived Works
  Modifications and derived works must be allowed, and must be distributed under the same terms as the license of the original software.
• Integrity of the Authors Source Code
  The distribution of modified source code must be allowed although restrictions to ensure the possibility to distinguish the original source code from the derived works are tolerated. For example, the possibility of using different software names.
• No Discrimination Against Persons or Groups
  The license must not discriminate against any person or group of persons.
• No Discrimination Against Fields of Endeavor
  The license must not restrict anyone from making use of the program in a specific field of endeavor. For example, it may not restrict the program from being used in a business, or from being used for genetic research.
• Distribution of License
  The rights attached to the program must apply to all to whom the program is redistributed without the need for execution of an additional license by those parties.
• License Must Not Be Specific to a Product
  The rights given by the license must not be different for the original distribution and any other one even when it takes place in a totally different context.
• License Must Not Contaminate Other Software
  The license must not place restrictions on other software that is distributed along with the licensed software. For example, the license must not insist that all other programs distributed on the same medium must be open-source software.