Background: Although autistic adults often discuss experiencing ''autistic burnout'' and attribute serious negative outcomes to it, the concept is almost completely absent from the academic and clinical literature. Methods: We used a community-based participatory research approach to conduct a thematic analysis of 19 interviews and 19 public Internet sources to understand and characterize autistic burnout. Interview participants were autistic adults who identified as having been professionally diagnosed with an autism spectrum condition. We conducted a thematic analysis, using a hybrid inductive-deductive approach, at semantic and latent levels, through a critical paradigm. We addressed trustworthiness through multiple coders, peer debriefing, and examination of contradictions. Results: Autistic adults described the primary characteristics of autistic burnout as chronic exhaustion, loss of skills, and reduced tolerance to stimulus. They described burnout as happening because of life stressors that added to the cumulative load they experienced, and barriers to support that created an inability to obtain relief from the load. These pressures caused expectations to outweigh abilities resulting in autistic burnout. Autistic adults described negative impacts on their health, capacity for independent living, and quality of life, including suicidal behavior. They also discussed a lack of empathy from neurotypical people and described acceptance and social support, time off/reduced expectations, and doing things in an autistic way/unmasking as associated in their experiences with recovery from autistic burnout. Conclusions: Autistic burnout appears to be a phenomenon distinct from occupational burnout or clinical depression. Better understanding autistic burnout could lead to ways to recognize, relieve, or prevent it, including highlighting the potential dangers of teaching autistic people to mask or camouflage their autistic traits, and including burnout education in suicide prevention programs. These findings highlight the need to reduce discrimination and stigma related to autism and disability. Internet sources (five in-depth). We recruited in the United States by publicizing on social media, by word of mouth, and through community connections. When analyzing interviews, we took what people said at face value and in deeper social context, and looked for strong themes across data.What were the results of the study?The primary characteristics of autistic burnout were chronic exhaustion, loss of skills, and reduced tolerance to stimulus. Participants described burnout as happening because of life stressors that added to the cumulative load they experienced, and barriers to support that created an inability to obtain relief from the load. These pressures caused expectations to outweigh abilities resulting in autistic burnout. From this we created a definition:Autistic burnout is a syndrome conceptualized as resulting from chronic life stress and a mismatch of expectations and abilities without adequate supports. It is ch...