The term 'Past Masters' refers to professionals and historians who have achieved a high level of expertise in their field. The crossword solutions for this term are PROS (4 letters) and HISTORIANS (10 letters). Professionals, or PROS, are individuals who have acquired a great deal of knowledge and experience in their chosen field. Historians are people who study the past and document events, people, and cultures. They often use primary sources such as documents, artifacts, and interviews to gain insight into the past.