histiocytosis

noun
his·​tio·​cy·​to·​sis | \ -ˈtō-səs How to pronounce histiocytosis (audio) \
plural histiocytoses\ -​ˌsēz How to pronounce histiocytoses (audio) \

Medical Definition of histiocytosis

: abnormal multiplication of macrophages Langerhans cell histiocytosis broadly : a condition characterized by such multiplication