This poster aims to clarify the notion of engagement in MOOCs discussion forums as perceived by students. The analysis of the 11 semi-structured interviews conducted with MOOC students highlights three main findings. First, engagement in MOOCs forums as a construct with four dimensions (behavioral, affective, cognitive and social). Secondly, a clear distinction between these dimensions using the related indicators we identified. Finally, the dynamic aspect of engagement. These results provide a socially and contextually grounded definition of engagement in MOOCs forums. That ensures the development of reliable and essential tools for measuring it and observing its quality over time.