PURPOSE:To analyze the plural definitions and applications of the term "hidden curriculum" within the medical education literature and to propose a conceptual framework for conducting future research on the topic. METHOD:The authors conducted a literature search of nine online databases, seeking articles published on the hidden, informal, or implicit curriculum in medical education prior to March 2017. Two reviewers independently screened articles with set inclusion criteria and performed kappa coefficient tests to evaluate interreviewer reliability. They extracted, coded, and analyzed key data, using grounded theory methodology. RESULTS:The authors uncovered 3,747 articles relating to the hidden curriculum in medical education. Of these, they selected 197 articles for full review. Use of the term "hidden curriculum" has expanded substantially since 2012. U.S. and Canadian medical schools are the focus of two-thirds of the empirical hidden curriculum studies; data from African and South American schools are nearly absent. Few quantitative techniques to measure the hidden curriculum exist. The "hidden curriculum" is understood as a mostly negative concept. Its definition varies widely, but can be understood via four conceptual boundaries: (1) institutional-organizational, (2) interpersonal-social, (3) contextual-cultural, and/or (4) motivational-psychological. CONCLUSIONS:Future medical education researchers should make clear the conceptual boundary or boundaries they are applying to the term "hidden curriculum," move away from general musings on its effects, and focus on specific methods for improving the powerful hidden curriculum.