Within educational and social discourses, the term 'inclusion' has various meanings. In both research and practice, there is no official definition of inclusion. Nevertheless, subjective definitions affect pedagogical acts. We developed the Definitions of Inclusion Questionnaire ["Fragebogen zur Erfassung subjektiver Definitionen von Inklusion"] (FEDI) as an economic instrument that takes subjective definitions of inclusion into account. This paper describes the construction and first psychometric evaluation of the questionnaire instrument that 513 participants with various professional backgrounds completed. We used exploratory and confirmatory factor analyses of two different subsamples to identify the measuring model. We found a three-factor structure with acceptable-to-good fit measures and an acceptable reliability ("CR" from 0.81 to 0.87). Small to medium correlations of the FEDI scales with attitudes towards inclusion and teachers' sense of efficacy suggest that discriminant validity is given. Perspectives for further research and implications for practice are discussed.