With the development of population ageing, the incidence of Alzheimer disease (AD) is increasing year by year, and the disease has become an important problem of public health due to its heavy social and economic burden. Owing to the lack of effective treatment in dementia period, the current research hotspots have moved gradually forward, and studies have focused on the prevention and treatment in the pre-dementia phase — mild cognitive impairment. Thus, a great many definitions have generated. The article reviewed the related concepts of AD at different stages, including preclinical stage, mild cognitive impairment and dementia, in which the clinicians can achieve a comprehensive acknowledge on the common types of cognitive decline and on the stage of development.