Final answer:
The prefix that denotes 'fever' in medical terminology is 'pyr/o'. It is used in terms such as 'pyrexia' for a febrile condition and 'antipyretic' for substances that reduce fever.
Step-by-step explanation:
The word part that means "fever" is pyr/o. This prefix is derived from the Greek word pyretos, meaning fever, and is commonly used in medical terminology. For example, pyrexia is a term that denotes a fever or febrile condition, and antipyretic refers to a substance that reduces fever.