Definition
Open source is software whose source code is published under a licence that grants any recipient the right to read it, modify it, and redistribute it — original or modified. The Open Source Initiative formalizes a set of criteria a licence must meet to qualify, but the practical idea is older and simpler: the recipe is shipped with the cake, and you are free to bake your own.
Open source overlaps but is not identical with free software in the Free Software Foundation's sense. Free software emphasizes the user's ethical freedoms ("free as in freedom"); open source emphasizes the pragmatic benefits of code-sharing — auditability, collaboration, and avoidance of vendor lock-in. In practice the codebases are largely the same; the labels reflect different framings of the same underlying licence regime.
Why it matters
How it works
A licence is the legal mechanism that turns a copyrighted work into shareable infrastructure. Without an explicit licence, copyright law defaults to "all rights reserved" — recipients have no right to copy, modify, or redistribute. An open-source licence is a grant from the copyright holder that explicitly extends those rights to anyone under stated terms. The two main licence families are permissive (MIT, BSD, Apache) and copyleft (GPL, LGPL, MPL). Permissive licences let you do nearly anything with the code, including using it in closed-source products. Copyleft licences require that derivative works also be open-source under the same terms — the freedom propagates.
The development model that grew up around these licences is as important as the licences themselves. A well-run open-source project has a public source repository, a public issue tracker, a public mailing list or chat, and a documented contribution process. Anyone can read the code, file an issue, submit a patch, or fork the project entirely. Maintainers act as gatekeepers but cannot prevent forks. The combination of permissive licensing and public collaboration is what lets a project like the Linux kernel sustain itself with contributors who do not share an employer, a country, or sometimes even an ideology.