One of the main goals of STEM (science, technology, engineering, mathematics, and medicine) education is the development of STEM identity, yet what STEM identity is remains elusive. This study examines how STEM professionals define themselves via open-ended responses to an online qualitative survey. Through inductive and deductive coding informed by sociocultural, professional, and STEM identity perspectives, supplemented by text mining, valid responses from 103 participants were qualitatively analyzed. Several main findings emerged: (1) STEM professionals primarily defined themselves by their STEM work-related duties; (2) they emphasized STEM competence through their educational background and expertise; (3) recognition by others was influenced by perceived performance and competence; and (4) STEM professionals aspired to advance and be at the forefront of STEM fields through continuously learning and making positive societal impacts. In addition to rethinking STEM education, supports, and public messaging about STEM and STEM professionals through an equity lens, implications include the development of STEM identity instruments that can be used to evaluate the effectiveness and inclusiveness of STEM education and educational contexts, programs, and reforms.