The term predates its use in software, but as used in the software industry it typically refers to the Open Source Definition: https://opensource.org/osd https:

The term predates its use in software, but as used in the software industry it typically refers to the Open Source Definition: https://opensource.org/osd https://opensource.org/osd Deviating from that isn’t illegal (they don’t own the term), but claiming your software is open source if it doesn’t meet that definition will generally earn you some blowback.