OBJECTIVES: To understand how patients with chronic non-cancer pain define and describe breakthrough pain (BTP). METHODS: This prospective study included outpatients from a large U.S. tertiary medical center who suffer from chronic pain due to headache, arthritis/rheumatism, musculoskeletal problems, or sickle cell anemia. Data were collected using a 1-week pain diary with questionnaire that captured their perceptions of BTP. Participants were asked to choose a term that best describes a pain flare and a definition of the term they selected. Pain scores were captured using a 10-point visual analog scale (VAS). RESULTS: The study cohort included 161 patients (36 with headache, 19 with arthritis/rheumatism, 17 with sickle cell anemia, 8 with musculoskeletal problems, and 70 with =2 pain conditions). Most were female (80.1%), white (67.7%), and experienced BTP during the diary week (90.5%). The mean pain level reported during the diary week was 6, and the mean age was 49.3 years. The terms used to describe BTP were “pain flare” (34%), “acute pain episode” (29.1%), “pain crisis” (19.9%), “sudden new pain episode” (16.5%), and “breakthrough pain” (11.9%). There were no differences by pain source except among headache patients, where more than half (52.8%) termed BTP as “an acute pain episode.” Most commonly selected definitions for BTP were “sudden pain more than your chronic pain” (31.3%), “a period of pain worse than your controlled pain” (26%) and “a brief episode of pain more intense than your usual pain” (21.3%). There were no differences in preferred BTP definition by pain source. CONCLUSION: Patients with chronic pain prefer to use the terms “pain flare” and “acute pain episode” rather than “breakthrough pain” when referring to BTP. The concepts of “brief” and “sudden” appear to be important when defining BTP. Results will be helpful to outcomes researchers who study pain.