Dementia is not a single disease, but a non-specific syndrome (i.e., set of signs and symptoms). Affected cognitive areas can be memory,attention, language, and problem solving. Normally, symptoms must be present for at least six months to support a diagnosis.^[3] Cognitive dysfunction of shorter duration is called delirium.

Especially in later stages of the condition, subjects may be disoriented in time (not knowing the day, week, or even year), in place (not knowing where they are), and in person (not knowing who they and/or others around them are).

Dementia can be classified as either reversible or irreversible, depending upon the etiology of the disease. Fewer than 10% of cases of dementia are due to causes that may be reversed with treatment.

Dementia is not merely a problem of memory. It reduces the ability to learn, reason, retain or recall past experience and there is also loss of patterns of thoughts, feelings and activities. Additional mental and behavioral problems often affect people who have dementia, and may influence quality of life, caregivers, and the need for institutionalization. As dementia worsens individuals may neglect themselves and may become disinhibited and may become incontinent. (Gelder et al. 2005). Behaviour may be disorganized, restless or inappropriate. Some people become restless or wander about by day and sometimes at night. When people with dementia are put in circumstances beyond their abilities, there may be a sudden change to tears or anger (a "catastrophic reaction").^[4] A common symptom of dementia for dementia sufferers to deny that relatives, even relatives in their immediate family, are their own relatives.