Background: Knowledge 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. Methods: We 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. Results: The 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. Conclusions: Knowing disability professionals' understandings of ableism is necessary to intervene biased attitudes and reduce ableism.