This paper introduces a new definition for burnout and investigates the psychometric properties of the Burnout Assessment Tool (BAT). In a prior qualitative study, 49 practitioners were interviewed about their conceptualization of burnout (part 1). Using a dialectical approach, four core dimensions—exhaustion, mental distance, and impaired emotional and cognitive impairment—and three secondary dimensions—depressed mood, psychological distress, and psychosomatic complaints—emerged, which constitute the basis of the BAT. In the second study, the psychometric characteristics of the BAT were investigated in a representative sample of 1500 Flemish employees, focusing on factorial validity, reliability, and construct validity, respectively. Results demonstrate the assumed four-factor structure for the core dimensions, which is best represented by one general burnout factor. Contrary to expectations, instead of a three-factor structure, a two-factor structure was found for the secondary dimensions. Furthermore, the BAT and its subscales show adequate reliability. Convergent validity and discriminant validity with other burnout measures—including the MBI and OLBI—was demonstrated, as well as discriminant validity with other well-being constructs, such as work engagement and workaholism.