Mild Cognitive Impairment (MCI) is now widely but inconsistently used to describe a broad range of cognitive states. While there may not yet be a universally accepted definition, there is adequate consensus that MCI describes the space on the cognitive continuum between "Normal" and "Demented."
Recall that one does not meet the clinical definition of "dementia" until their impairment is severe enough to interfere with their social or occupational function. From this definition, the need has arisen to describe the situation when a person is beginning to lose thinking ability but their impairment is still subtle and not yet severe enough to meet the criteria for "dementia." To address that need, researchers at the Mayo Clinic constructed the term Mild Cognitive Impairment, which has been quickly, if not uniformly, adopted in the scientific community.
As with dementia, Mild Cognitive Impairment is not a diagnosis and not something that must be treated. It is merely a symptom of some underlying medical condition such as depression, Parkinson's disease, anxiety, stroke, vitamin B-12 deficiency, early stage Alzheimer's disease, etc.