Definition. Software whose source code is published under a licence that permits use, study, modification and redistribution, in line with the OSI's Open Source Definition. Well-known licences are GPL, MIT, Apache-2.0 and MPL. “Open source” on its own says nothing about who governs or funds the project.

Why it matters. For mid-sized companies, open source is mostly a question of long-term ability to act. If a vendor fails, changes its licence or changes its strategy, the code remains accessible and can be taken over. That is no guarantee of quality, but it is a safety net.

Related. Open Source CMS, Digital Sovereignty, Vendor Lock-in, Self-hosted Infrastructure