Although commonly described on social media by autistic people, there is little recognition of autistic burnout in the academic literature. Anecdotally, autistic burnout is described as a debilitating condition that severely impacts functioning, is linked to suicidal ideation and is driven by the stress of masking and living in an unaccommodating neurotypical world. We sought to define autistic burnout using the Grounded Delphi method. Autistic adults, experts by the lived experience of autistic burnout (n = 23), co-produced and agreed to a definition intended for clinicians and the autistic and autism communities. A thick description and conceptual framework were developed from the open-ended round 1 survey, with a high majority of agreement reached in the round 3 survey. Autistic burnout was defined as a highly debilitating condition characterised by exhaustion, withdrawal, executive function problems and generally reduced functioning, with increased manifestation of autistic traits -- and distinct from depression and non-autistic burnout. Further work is needed to differentiate autistic burnout from other conditions and to build clinician understanding of the accompanying complexity to be considered in treatment planning.