A more thorough explanation:

A decretist is a type of student in medieval universities who studies law, specifically Gratian's Decretum. They may also be a commentator on this text.

For example, a student who is studying law in the Middle Ages and focusing on Gratian's Decretum would be considered a decretist. They would spend their time analyzing and interpreting this text, as well as discussing it with other students and scholars.

The term "decretist" comes from the Latin word "decretum," which means "decree" or "judgment." This reflects the focus of the decretist's studies, which involve understanding and interpreting legal decrees and judgments.