fever vs pyrexia

fever and pyrexia both are nouns.

Word	Noun	Adjective	Verb	Adverb
fever	Yes	No	No	No
pyrexia	Yes	No	No	No

As nouns, fever and pyrexia are synonyms defined as:

fever and pyrexia: a rise in the temperature of the body; frequently a symptom of infection

Other synonyms of fever include febricity, febrility, feverishness.

fever (noun)	pyrexia (noun)
a state of nervous excitement	a rise in the temperature of the body; frequently a symptom of infection
a rise in the temperature of the body; frequently a symptom of infection