An endotype is a subtype of a health condition, which is defined by a distinct functional or pathobiological mechanism.
[1] This is distinct from a phenotype, which is any observable characteristic or trait of a disease, such as development, biochemical or physiological properties without any implication of a mechanism.
It is envisaged that patients with a specific endotype present themselves within phenotypic clusters of diseases.
Other authors simply continue with the classification of Whitbeck, leaving just three kinds of definition (clinical, pathological and aetiological).
[6] It is important to note that a real-world definition is normally a hybrid between these above kinds, and an endotype should use all three of the descriptors - including aetiology - to ensure specificity.