Autistic burnout is an experience commonly described by autistic people (#AutBurnout and #AutisticBurnout on social media). Recently, two definitions of this syndrome have been published. Both describe debilitating exhaustion with onset related to various stressors including masking, though several differences exist, such as the characteristic of interpersonal withdrawal. We sought to explore the content validity of these definitions including duration and frequency criteria, using descriptive statistics, content analysis and reflexive thematic analysis. A co-produced survey of 141 autistic adults with experience of autistic burnout showed strong endorsement of the definition by Higgins et al., where exhaustion and interpersonal withdrawal occur alongside reduced functioning, executive functioning difficulties, and increased manifestation of autistic traits. Duration and frequency criteria were unresolved, with qualitative data highlighting varying (both acute and chronic) experiences. Autistic burnout is frequently misdiagnosed as depression, anxiety, bipolar disorder, borderline personality disorder or other conditions. Work is needed to increase community and clinician awareness, as well as initiatives to improve unaccommodating neurotypical environments. More research and validation are needed in larger samples not restricted to autistic adults who have experienced autistic burnout to determine prevalence and risk factors as well as duration and frequency.