The Axon platform consists of Axon Framework and Axon Server. It is provided by AxonIQ B.V. under the licenses described below.
Axon Framework, as well as the Axon documentation and sample programs, are open source and provided under the Apache License v2.0.
For Axon Server, licensing depends on the version.
The standard version, called "Axon Server", is open source and free to download and use. It is provided under an AxonIQ-specific open source license. While this license allows you to run the software freely in any environment, it is less permissive than the Apache License. For instance, it doesn't allow you as a licensee to create a derivative work.
The commercial version, called "Axon Server Enterprise", as well as the various "Packs", are closed source and provided under a commercial license agreement.