diagnosis

First recorded in 1675–85; from New Latin, from Greek diágnōsis “a distinguishing, means or power of discernment”; see dia-, -gnosis

Although diagnosis and prognosis are both very familiar medical terms, they are sometimes used interchangeably (by nonmedical people), as if they were synonymous words. They are not.
When a person is unwell and asks a doctor what is wrong with them, what they are asking for is a diagnosis. Examining the symptoms, as with a thermometer or stethoscope, and evaluating the results of a procedure such as a throat culture, blood test, or x-ray will help the doctor identify the patient’s illness. Whatever that ailment may be, the identification, or naming of it, is the diagnosis. The diagnosis of rheumatoid arthritis was very disheartening. The diagnosis is an acute case of tendinitis.
Once a patient has been given a diagnosis, the next thing they’ll want to know is what sort of an outcome they can expect, which means they want to know the prognosis. Unlike diagnoses, which are conclusively based on tangible evidence, prognoses are reasonable predictions based on past observations of similar cases. A prognosis gives the patient an idea of what to expect about the course of their illness, including the probability of recovery. As ailments range from the most superficial to the most life-threatening, it is typical for the word prognosis to be qualified by such adjectives as excellent, favorable, good, positive, negative, poor, dire, grim. Doctors will never tire of telling us, “Early detection and treatment are your best bets for a positive prognosis.” And no one wants to say, “The prognosis was so poor that he could no longer hide his condition from his family.”
In nonmedical contexts, diagnosis still carries its meaning of naming or identifying something, especially when that involves a situation or problem. When our computer kept crashing, the tech said that nearby UFO activity could be the reason, so we got another tech—and a proper, earthbound diagnosis!
Likewise, prognosis is used outside of medical contexts, retaining its meaning of reasonable prediction. As a lover of pesticide-free smoothies, I’m pleased to say that the prognosis for the future of organic farming has never been better.