Mononucleosis is a condition where there is an unusual proliferation of lymphocytes in the blood due to an infection with the Epstein-Barr virus (EBV). These atypical lymphocytes resembled “monocytes” when they were first discovered; thus the term mononucleosis was coined.

The term may refer to:

EBV infectious mononucleosis known colloquially as the “kissing disease,” caused by Epstein-Barr virus