Computer programs made available for use, modification, and distribution without express approval from the developer.
An open-source software (OSS) is one whose developer releases the authoritative rights on it by allowing other people to obtain, modify, and even distribute it for free. These kinds of programs have their source codes published and are usually geared towards boosting increased activity in a community of users. Moreover, these programs are usually developed by a large number of individuals and require the involvement of a community of others to oversee its progression over the years.
There are different kinds of OSS. Some of them allow free acquisition, use, modification, and distribution. Others are only modified and distributed on the condition that the original developer license is affixed on subsequent versions of the software.