It is difficult to establish clear distinctions between emotion, affect and mood, because they belong to the same physiological dimension which may be conceived as ‘background activity’; for the purposes of research definitions should be as precise as possible. After a review of the most commonly used and sometimes contradictory definitions of these and related terms, a modified version of Janzarik’s model of the ‘structural-dynamic coherency’ is proposed as a frame of reference for improvement in defining different ‘dynamic states’. The clear distinction between emotional resonance, mood and drive for the description of pathological modifications of the background activity