BackgroundKnowledge about how disability professionals understand ableism may provide insight into the production of inequalities. The aim of this study was to examine how disability professionals understand ableism.MethodsWe asked 347 disability professionals, all of whom worked with people with intellectual and developmental disabilities, among other populations, to define ableism and then analysed those definitions using content analysis.ResultsThe themes about how participants understood ableism were: discrimination; differential treatment; individualization; norms and othering; ableist language; microaggressions; and systems and environments. It was also not uncommon for participants to say ableist things, and express misconceptions in their definitions. This included these themes: avoiding disability; using ableist language; framing disability as in/ability; centring people without disabilities; ignoring invisible disabilities; believing only people without disabilities have bias; and believing ableism does not exist.ConclusionsKnowing disability professionals' understandings of ableism is necessary to intervene biased attitudes and reduce ableism.