Dementia (from the Latin for "without mind") describes any negative effect on brain function, such as memory, attention, language, and problem-solving. A typical example is when a person is unable to recognize people or surroundings. It may be permanent or the patient may have periods of lucidity. In patients with severe dementia, they may not be able to recognize even things that are extremely familiar, such as their home, spouse, or children.
It was once believed that dementia was simply a normal complication of aging. However, it's currently believed that, although certain cognitive functions may be impaired with age, in a normal human brain, those functions degrade slowly and only to a very slight degree. Dementia is only used to describe cognitive function which is well below that of a person of a similar age. Although dementia usually affects only people over the age of 65, it can strike at any age.
Dementia is a serious symptom and it usually indicates a serious underlying condition. Dementia patients may also appear to be lucid, but it quickly becomes apparent that they have no clear knowledge of their surroundings, or that they intend to do something that is impossible (like walking to something that is in another city or visiting a dead relative at their old home). They often have to be watched constantly to prevent them from inadvertently hurting themselves. However, because of their condition, they usually cannot understand why physicians or nurses are interested in them.
Dementia must be distinguished from other conditions. For example, a patient suffering from a concussion may be confused or get facts wrong, but will be able to respond appropriately to questions, although they might get the facts wrong (i.e. when asked what day of the week it is, they will respond with one of the seven days of the week, but just not the right one). They will also be able to identify their surroundings and close acquaintances. Similarly, sufferers of Korsakoff's syndrome may not be able to remember anything, but will come up with appropriate responses to questions, even though they are confabulating the answers. Dementia patients will not be able to respond appropriately to questions.
One of the best known diseases that causes dementia is Alzheimer's disease. In House, Eric Foreman's mother suffers from occasional dementia, and she once believed her adult son was still a small boy, even though he was standing right in front of her.