Where you might see or hear it: If you go to see a healthcare provider because you are having symptoms such as a sore throat and a headache, they will ask you questions and might do some tests to figure out what is causing you to feel unwell. The answer that they come up with is their "diagnosis."
The process of making a diagnosis is not always straightforward. Sometimes, doctors come up with many possibilities first (a differential diagnosis) then narrow it down to a few that are the most likely. If they are pretty sure, but still waiting for more definitive proof, they might assign a person a provisional diagnosis.
When you might want to use it: Sometimes, a diagnosis is not forever. For example, if you have a case of the flu, you'll get better in a matter of weeks at most. In other cases, a diagnosis is more permanent, such as if you find out that you have a condition like lupus that is lifelong (chronic).
If you have been feeling unwell for a long time and a doctor is finally able to figure out what is causing your symptoms, you might feel relieved to be able to tell your loved ones that you have a diagnosis—even if it will change your life or present some challenges for you.