What is the definition of a fever of unknown origin and how is it evaluated?

Bottom line:  Fever of unknown origin (FUO) can be defined as a temperature > 38.3°C on several occasions and lasting longer than 3 weeks, with a diagnosis that remains uncertain after 1 week of investigation.

Summary:  Mourad O, et al.  A comprehensive evidence-based approach to fever of unknown origin.  Arch Intern Med. 2003 Mar 10;163(5):545-51.
This systematic review summarizes evidence on prevalence of various diagnoses in patients presenting with FUO (no diagnosis, infection and inflammatory causes were most common), as well as evidence supporting use of various diagnostic tests in the work-up of FUO, including abdominal CT, Duke criteria, temporal arterery biopsy, nuclear imaging, etc., and providing an overview of basic diagnostic work-up (see Table 1.)