The Process Engine REST API is a supported equivalent of the Activiti Open Source API. This means that all operations described in the Activiti User Guide [1] are available as documented there, except for REST endpoints that are not relevant for the enterprise product (for example, forms, as they are implemented differently).
This REST API is available on <your-server-and-context-root>/api/
<your-server-and-context-root>/api/repository/process-definitions
For example <your-server-and-context-root>/api/repository/process-definitions?tenantId=tenant_1
Links:
[1] http://activiti.org/userguide/index.html#_rest_api
[2] https://docs.alfresco.com/../topics/rest_api.html