What is the definition of prolonged?

ANSWER: Prolonged simply means that something is lengthened in its extent. You can have the flu that was "PROLONGED" due to having a weak immune system. Or you could have a prolonged wait at the airport due to a delayed aircraft.

it could mean that the body temperature does keeps fluctuating between 99-103f for a long period of time say more than a week .

Definition According to Merriam-Webster, the definition of prolong (the present tense of prolonged) is: 1 : to lengthen in time : 2 : to lengthen in extent, scope, or range
It means "lasting for a long time".