There is no single and universal definition for Open Source Software (OSS). The simplest definition would be software for which the source is freely available, meaning you do not need to license or buy it to use it legally (thus leaked source code does not make a program open source). This lack of clarity is reflected in the number of open source licenses available. There are nearly seventy open source licenses as classified by the Open Source Initiative (OSI), which has a relatively strict definition. There are countless more sort of open source licenses which may not meet a fully open source criteria for any number of reasons (and thus should not really be considered open source licenses). The Open Source Initiative is, according to its website, "a non-profit corporation with global scope formed to educate about and advocate for the benefits of open source, and to build bridges among different constituencies in the open source community."

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