[ks]

It is open sourced under Microsoft Public License: https://github.com/BabylonJS/Babylon.js

According to the Free Software Foundation, it is a free software license but not compatible with the GNU GPL.

[fhd2]

> but not compatible with the GNU GPL.

That doesn't mean much, the GPL (v3) is incompatible with itself (v2). Not a GPL hater, just pointing out that it's not a big deal for licenses to be incompatible with the GPL.

[swang]

Yes you're right about that, although this is only mentioned in the blog post, not the website. I'm also guessing this is open source the same way say Google open sources Android or Apple open sources Darwin.