A diagnosis comes first: it identifies what the problem is. A prognosis comes after: it predicts how that problem is likely to develop. Think of the order they happen in a doctor's visit, and you will never mix them up.
The core difference
A diagnosis names the condition; a prognosis forecasts its course.
- diagnosis — identifying a disease or the cause of a problem: The diagnosis confirmed what the family had feared for months.
- prognosis — a prediction of how a condition or situation will develop: The doctor gave a cautiously optimistic prognosis after surgery.
How to tell them apart
Both words trace to Greek roots built on gnosis, "knowledge," which is exactly why they get confused. The trick is the prefix: dia- means "through," as in seeing through to what is really going on now; pro- means "before," as in looking ahead to what comes next. Diagnosis looks at the present; prognosis looks at the future.
Outside medicine, both words extend naturally to any problem. An IT team can give a diagnosis of a network outage, and an economist can offer a prognosis for the housing market — the same before-and-after order still applies.
