The ecosystem is an essential biological concept that represents the major structural and functional unit in nature. Here, I propose seven hallmarks that characterize any ecosystem: biodiversity, habitat, hierarchy, interactivity, openness, homecostasis, and evolutionary. In this set of characteristics, there are two main singularities: the inclusion of viruses as fundamental component of biodiversity, and the coining of a new term, homecostasis, to refer to the maintenance of the stability of ecosystems in the face of disturbances. From the ensemble of these hallmarks, I define the ecosystem as a set of evolutionary living and environmental worlds that functions, in a dynamic equilibrium, as an open, interactive, and hierarchical system. Furthermore, I show an alternative way of graphically representing the ecosystem concept as a network of interconnections between the four worlds. Finally, I propose an alternative configuration of the tree of life, taking the ecosystem as the core from which all living worlds emerge.