Open Source refers to something people can modify and share because its design is publicly accessible.

Open-source software (OSS) is computer software with its source code made available with a license in which the copyright holder provides the rights to study, change, and distribute the software to anyone and for any purpose. Open-source software may be developed in a collaborative public manner. According to scientists who studied it, open-source software is a prominent example of open collaboration.
Open-source software Wikipedia Entry

Official Open Source Software Definition as defined by the Open Source Initiative (OSI)

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