A stable version of definitions package was tagged.
The package provides syntax constructs describing a way to create and configure a service or an object.
The following are provided:

• Definitions describing services or objects to create. This includes syntax, its validation and resolving it to objects.
• References and dynamic references to point to other definitions. These include additional utility to refer to multiple
  definitions at once.

While, by itself, the package is not very useful for end users of the framework, it is a base upon which yiisoft/di and yiisoft/factory are built. And the base is solid with a great code coverage and mutation score.