diploma; degree certificate
Refers specifically to the physical certificate issued upon graduation, distinct from the degree itself (学位).
I received my diploma at the graduation ceremony.
卒業証書 is a general graduation certificate, often used for high school or below. 学位記 is specifically for a degree (university level or higher).
Compound of 学位 (がくい, 'academic degree') + 記 (き, 'record' or 'document').