Pyrexia, commonly known as fever, can be described using various synonymous terms. Some other terms used to define pyrexia include febrility, febricity, and hyperthermia. Pyrexia is characterized by an increase in body temperature above the normal range of 36-37 degrees Celsius (98-100 degrees Fahrenheit). It is a response of the immune system to infections, diseases or other inflammatory conditions in the body. Other synonyms of this condition include feverishness, a rise in body heat, high temperature, and overheating. Fever can be caused by a variety of reasons and identifying the underlying cause of pyrexia is crucial in effective treatment of the condition.