Background: The Outcome Measures in Rheumatology (OMERACT) Glucocorticoid (GC) Impact Working Group has been working to develop a core domain set to measure the impact of GCs on patients living with rheumatic and musculoskeletal diseases. The mandatory domains previously identified for inclusion in all clinical trials measuring the GC effects include infection, bone fragility, mood disturbance, hypertension, diabetes, weight, fatigue, and mortality. Before progressing to instrument selection, the Working Group sought to establish precise definitions of all mandatory domains within the core domain set. Methods: OMERACT methodology was applied with the use of evidence and consensus-based decision making of all stakeholder groups (patient research partners, health care professionals, clinician researchers, industry members and methodologists) to develop detailed definitions for the broad domain, target domain and domain components, taking into consideration sources of variability that could affect measurement of the domain. The working group synthesized prior qualitative studies, quantitative work, and results from Delphi rounds, to develop a rich definition of ‘what’ is to be measured. Results: Between 2021 and 2023, the OMERACT Working Group on GC Impact conducted virtual meetings to establish domain definitions. First, we mapped each domain onto an OMERACT Core Area. All domains were primarily represented within the Pathophysiological Manifestations Core Area, except from Fatigue which was primarily Life Impact and Weight which spanned both Core Areas. Sources of variability included cultural factors, age, gender, education level, socioeconomic status, personal experiences, emotional state, and language barriers. The domain definitions will form the foundation for instrument selection and the initial step of domain / concept match and content validity in the OMERACT pillar of ‘truth’ before moving on to feasibility and discrimination. Conclusion: The OMERACT GC Impact Working Group has developed and agreed upon detailed domain definitions for core domains. Future steps of the working group are to select instruments and develop the core outcome measurement set for clinical trials measuring the impact of GC on patients with rheumatic and musculoskeletal diseases.