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Multimorbidity, also known as multiple-long term conditions (MLTC), means living with two or more chronic illnesses. For example, a person could have diabetes, heart disease and depression. Multimorbidity is common in older adults, estimated to affect over half of those aged 65 and over. This increased prevalence has been explained by older adults' "longer exposure and increased vulnerability to risk factors for chronic health problems". The prevalence of multimorbidity has been increasing in recent decades. The high prevalence of multimorbidity has led to some describing it as "The most common chronic condition". Multimorbidity is also more common among people from lower Socioeconomic statuses. Multimorbidity is a significant issue in low‐ and middle‐income countries, though prevalence is not as high as in high income countries.

Definition and names
The concept of multiple long-term conditions is burdened by a lack of widely agreed upon definition and by being referred to by various names. It is also often used synonymously with the related term comorbidity even though the two are considered distinct clinical scenarios.

Difference from comorbidity
Comorbidity means that out of a person's multiple conditions one is in the focus of attention (the index condition) and others are viewed in their relation to the 'main' disease. In contrast, multimorbidity describes someone having two or more long-term (chronic) conditions without any of them holding priority over the others. This distinction is important in how the healthcare system treats people and helps making clear the specific settings in which the use of one or the other term can be preferred. Multimorbidity offers a more general and person-centered concept which allows focusing on all of the patient's symptoms and providing a more holistic care. In other settings, for example in pharmaceutical research, comorbidity might often be the more useful term to use.

Definitions
The broad definition of multimorbidity, consistent with what is used by most researchers, the WHO and the UK's Academy of Medical Sciences is the "co-existence of two or more chronic conditions". These can be physical non-communicable diseases, infectious and mental health conditions in any possible combinations and they may or may not interact with each other. When the co-existing conditions have similar origins or treatments the terms used is concordant multimorbidity, while discordant multimorbidity is used to refer to conditions that appear to be unrelated to each other.

Definitions of multimorbidity usually differ in the minimum number of concurrent conditions they require (most often this is two or more) and in the types of conditions they consider. For example the UK's National Institute for Health and Care Excellence (NICE) includes alcohol and substance misuse in their list of conditions considered to constitute multimorbidity.

The most commonly used term to describe the concept is multimorbidity. However, scientific literature shows a diverse range of terms used with the same meaning. These include comorbidity, polymorbidity, polypathology, pluripathology, multipathology, multicondition.

The National Institute for Health and Care Research (NIHR) uses the term multiple long-term conditions (MLTC) as it is more accepted and understood by patients and the public.