noun
medical certificate; doctor's note
A formal document issued by a doctor or medical institution stating a diagnosis, condition, or fitness for work/school.
会社を休むために、病院で診断書をもらった。
I got a medical certificate from the hospital to take time off work.
学校に診断書を提出してください。
Please submit a doctor's note to the school.
証明書 is a general certificate or proof document; 診断書 is specifically a medical certificate.
Compound of 診断 (diagnosis) + 書 (document).