Adversariality of the agents with respect to the multi-agent system can be a serious issue in the design of open multi-agent systems. Until now, many incoherent definitions of such behavior were used, preventing the consolidation of the knowledge about the domain. By basing ourselves on the valid and accepted results from economics, law and conflict theory, we propose a consistent definition of adversariality in the multi-agent systems and discuss the characteristics of the behavior that falls into this definition.