Dementia is a clinical term for a group of conditions causing progressive decline in memory and thinking. Senility is an old informal word implying that decline is simply what old age does. Medicine abandoned senility precisely because that assumption is wrong — dementia is disease, not ageing.
The core difference
- dementia — an umbrella term for conditions causing progressive loss of memory, reasoning and function, severe enough to affect daily life.
- senility — a dated, non-clinical word for mental decline attributed to old age itself.
Why senility was dropped
The word encodes a false premise: that losing your memory is a normal part of getting old. It is not. Most older people do not develop dementia, and dementia has identifiable disease processes behind it. Treating decline as inevitable delayed diagnosis and discouraged investigation, which is why the term fell out of clinical use.
It survives in older writing and in the phrase senile dementia, a historical label for dementia beginning after 65 — a distinction now abandoned, since the same diseases occur earlier.
The main types
- Alzheimer's disease — the most common cause, roughly two-thirds of cases.
- Vascular dementia — caused by reduced blood supply to the brain, often after strokes.
- Lewy body dementia — associated with visual hallucinations and movement symptoms.
- Frontotemporal dementia — often begins with changes in personality or language rather than memory, and typically starts younger.
Dementia is not a diagnosis on its own
It describes a syndrome — a pattern of symptoms — rather than a single disease, which is why a diagnosis normally names the underlying cause. Some causes of dementia-like symptoms are treatable or reversible, including certain vitamin deficiencies, thyroid problems and medication effects, which is another reason the fatalism of senility was unhelpful.
Where the words come from
Dementia is Latin: de- (away from) plus mens (mind) — out of one's mind. Senile is from Latin senex, an old man, the same root as senator and senior. Originally it meant nothing more than aged.
This page explains the words, not your symptoms. Anything of concern is a question for a doctor.
Frequently asked questions
Is senility the same as dementia?
No. Dementia is a clinical term for diseases causing progressive decline. Senility is an old word assuming decline is simply part of ageing, which is why medicine stopped using it.
Is dementia a normal part of ageing?
No. Most older people do not develop dementia. It is caused by identifiable disease processes rather than age itself.
Is dementia a single disease?
No, it is a syndrome. Alzheimer's disease is the most common cause, followed by vascular, Lewy body and frontotemporal dementia.