Cancer management requires accurate identification of the causes of treatment failure followed by cause-specific interventions. One cause of treatment failure that has received too little attention is abandonment of treatment. Abandonment includes 2 key elements: (1) failing to complete therapy for a disease that could be cured or definitively controlled, and (2) missing all treatment for a sustained time period to an extent that impacts the ability to cure or definitively control the disease. In this Historical Insight paper, we analyze historic and current terminology used to describe treatment abandonment in patients and provide a framework to distinguish it from nonadherence and loss to follow-up.