Diagnosis
Diagnosing, Assessment
noun, の adjective, verbal noun
To diagnose is to (metaphorically) cut off all the possibilities you don't believe to be likely. That's how you get a diagnosis!
診断 is primarily used in medical contexts to mean the act of diagnosing or the diagnosis itself. In non-medical contexts, it can also mean an assessment used to evaluate something, like a 性格診断 (personality test).
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
どうしてその診断結果になったのか、診断のプロセスを説明してもらえませんか？
Could you explain the process of how you came to that diagnosis?
MBTI診断っていう性格診断テスト、知ってる？
Do you know the personality test called MBTI assessment?
西村医師は乳がんの診断においては右に出る者はいないと言われています。
Doctor Nishimura is believed to be the best doctor in terms of their diagnosis of breast cancer.
私の息子は四歳の時に喘息だと診断されました。
My son was diagnosed with asthma when he was four.