As in other kinds of software development, the definition of process models in Multi-agent System (MAS) domain has many advantages. Some of these advantages are the better understanding which facilitates process measurement and improvement, and that the definition constitutes the basis for automating the process itself. The main goal of this paper is to provide a proper mechanism for defining agent-based development process models. For achieving this goal, an open-source editor tool and a technique for defining process models with the mentioned tool are presented. Both the editor tool and the technique provide MAS designers with a suitable mechanism for defining process models and are based on the Software Process Engineering Metamodel (SPEM). Although the main goal is to define process models for agent-based development, the proposed tool and technique can define any software process model, because they are based on a general-purpose software process metamodel. The utility of the tool and the technique is also justified, finally, by qualitative comparison with others.