Software Ecosystems (SECO) represent a dynamic interaction of actors, artifacts, and platforms, fostering software development and collaboration. Among these, proprietary SECO (PSECO) stand out due to their centralized management, intellectual property protection, and confidentiality agreements. Drawing inspiration from natural ecosystems, this work highlights the interdependence and balance required in PSECO, where changes introduced by the keystone can ripple across the ecosystem, impacting stability and scalability. This work aims to frame the concept of PSECO by analyzing and synthesizing term definitions, actors, characteristics, governance conditioning factors, and challenges related to PSECO. We performed a systematic mapping study, in which 32 were selected and analyzed to answer the research questions. Our work revealed the evolving complexity of PSECO by highlighting the growing adoption of standardized definitions, the role of key actors such as keystones and customers, the emergence of suppliers in mature ecosystems, the need for a holistic approach to interrelated dimensions, and the impact of governance conditioning factors on key PSECO characteristics.