Comorbidity refers to the presence of two or more conditions in an individual, and it is associated with worse health outcomes, more complex clinical management, and increased healthcare costs. However, there is no consensus on the definition of comorbidity or related concepts such as multimorbidity, morbidity burden, and patient complexity. This article reviews definitions of comorbidity and their relationship to these constructs, emphasizing the importance of precise terminology for research and clinical care. The article discusses the mechanisms underlying the coexistence of multiple conditions, including direct causation, associated risk factors, heterogeneity, and independence. It also considers the implications of these coexisting conditions for clinical care, epidemiology, and health services planning. The article highlights the need for internationally accepted definitions to advance research on comorbidity. The article also explores different conceptualizations of comorbidity, including morbidity burden and patient complexity. Morbidity burden refers to the overall impact of different diseases on an individual, while patient complexity considers not only health-related characteristics but also socioeconomic, cultural, environmental, and behavioral factors. The article integrates different constructs, such as comorbidity, multimorbidity, morbidity burden, and patient complexity, to illustrate their relevance in clinical care, epidemiology, and health services planning. It emphasizes the importance of using explicit definitions in conjunction with established disease classification systems to improve the precision and generalizability of findings. The article also discusses the relationships between comorbid diseases, including the potential for direct causation, associated risk factors, heterogeneity, and independence. It highlights the impact of comorbidity on disease management and the need for further research into patients' perspectives on how multiple conditions affect their health, well-being, and clinical care.Comorbidity refers to the presence of two or more conditions in an individual, and it is associated with worse health outcomes, more complex clinical management, and increased healthcare costs. However, there is no consensus on the definition of comorbidity or related concepts such as multimorbidity, morbidity burden, and patient complexity. This article reviews definitions of comorbidity and their relationship to these constructs, emphasizing the importance of precise terminology for research and clinical care. The article discusses the mechanisms underlying the coexistence of multiple conditions, including direct causation, associated risk factors, heterogeneity, and independence. It also considers the implications of these coexisting conditions for clinical care, epidemiology, and health services planning. The article highlights the need for internationally accepted definitions to advance research on comorbidity. The article also explores different conceptualizations of comorbidity, including morbidity burden and patient complexity. Morbidity burden refers to the overall impact of different diseases on an individual, while patient complexity considers not only health-related characteristics but also socioeconomic, cultural, environmental, and behavioral factors. The article integrates different constructs, such as comorbidity, multimorbidity, morbidity burden, and patient complexity, to illustrate their relevance in clinical care, epidemiology, and health services planning. It emphasizes the importance of using explicit definitions in conjunction with established disease classification systems to improve the precision and generalizability of findings. The article also discusses the relationships between comorbid diseases, including the potential for direct causation, associated risk factors, heterogeneity, and independence. It highlights the impact of comorbidity on disease management and the need for further research into patients' perspectives on how multiple conditions affect their health, well-being, and clinical care.