Delirium, an acute brain dysfunction, frequently affects intensive care unit (ICU) patients during the course of a critical illness. Besides the acute morbidities, ICU survivors often experience long-term sequelae in the form of cognitive impairment (LTCI-CI). Though delirium and LTCI-CI are associated with adverse outcomes, little is known on the terminology used to define these acute and chronic co-morbidities. The use of a correct terminology is a key factor to spread the knowledge on clinical conditions. Therefore, we first review the epidemiology, definition of delirium and its related terminology. Second, we report on the epidemiology of LTCI-CI and compare its definition to other forms of cognitive impairments. In particular, we define mild cognitive impairment, dementia and finally postoperative cognitive dysfunction. Future research is needed to interpret the trajectories of LTCI-CI, to differentiate it from neurodegenerative diseases and to provide a formal disease classification.