A guinea pig is a small rodent of the family Caviidae, and is often used as a test SUBJECT in scientific experiments. The term 'guinea pig' is also used to refer to a person who is used as a test SUBJECT, such as in medical or psychological experiments. The term 'lab rat' is also used to refer to a person who is used as a test SUBJECT in a laboratory setting. Finally, the term 'CAVY' is used to refer to the species of rodent that guinea pigs belong to.