Background Older people and people with dementia experience a high prevalence of multiple health conditions. The terms ‘comorbidity’ and ‘multimorbidity’ are often used interchangeably to describe this, however there are key conceptual differences between these terms and their definitions. This has led to issues in the validity and comparability of research findings, potentially inappropriate intervention development and differences in quality of health care. Objective To review how the terms ‘comorbidity’ and ‘multimorbidity’ are defined within peer-reviewed dementia research and propose an operational framework. Design A scoping review of definitions within dementia research was carried out. Searches took place across five databases: Academic Search Premier, CINAHL Complete, MEDLINE, PsycARTICLES and PsycINFO. PRISMA-ScR guidelines were followed. Results Content analysis revealed five key themes, showing significant overlap and inconsistencies from both within, and between, the comorbidity and multimorbidity definitions; 1. Number of conditions; 2. Type of health conditions; 3. The co-occurrence of conditions; 4. The inclusion of an index disease (or not); 5. Use of medical language. The analysis also revealed gaps in how the underlying concepts of the definitions relate to people with dementia living with multiple health conditions. Conclusion This scoping review found that current definitions of comorbidity and multimorbidity are heterogeneous, reductionist and disease-focussed. Recommendations are made on the design of research studies including transparency and consistency of any terms and definitions used. A syndemic framework could be a useful tool for researchers, clinicians and policy makers to consider a more holistic picture of a person with dementia’s health and wellbeing.

The growing number of patients with complex clinical profiles related to chronic diseases has contributed to the increasingly widespread use of the term 'multimorbidity'. A suitable measurement of this condition is essential to epidemiological studies considering that it represents a challenge for the clinical management of patients as well as for health systems and epidemiological investigations. In this context, the present essay reviews the conceptual proposals behind the measurement of multimorbidity including the epidemiological and methodological challenges it involves. We discuss classical definitions of comorbidity, how they differ from the concept of multimorbidity, and their roles in epidemiological studies. The various conceptual models that contribute to the operational definitions and strategies to measure this variable are also presented. The discussion enabled us to identify a significant gap between the modern conceptual development of multimorbidity and the operational definitions. This gap exists despite the theoretical developments that have occurred in the classical concept of comorbidity to arrive to the modern and multidimensional conception of multimorbidty. Measurement strategies, however, have not kept pace with this advance. Therefore, new methodological proposals need to be developed in order to obtain information regarding the actual impact on individuals' health and its implications for public health.