The number of interdisciplinary courses being developed in higher education across different countries is increasing. The people responsible for developing and enacting interdisciplinary courses can have significant design and teaching autonomy, but little is known about how they understand interdisciplinarity. This study uses phenomenographic interviews to address the question: how do interdisciplinary course leaders understand interdisciplinarity? This includes exploration of the relationships between different understandings of interdisciplinarity. Findings from interviews with 23 interdisciplinary course leaders reveal understandings of interdisciplinarity expressed in relation to (A) definitions, (B) knowledge spaces, and (C) situated knowledge work practices. Understandings were multiple and heterogeneous; they varied between participants and more importantly participants each expressed understandings belonging to multiple categories. Findings show that academics' views about what interdisciplinarity means are rarely exclusively aligned with normative definitions, but instead are marked by individuals' construction of meaning through processes of acquisition, sense-making and situational judgment. The study's findings could be used in future educational design and planning work to elicit and discuss different stakeholder understandings of interdisciplinarity.