Cambridge Dictionary: 

hyponym

noun [ C ]   LANGUAGE   specialized

US  /ˈhaɪ.pə.nɪm/ UK  /ˈhaɪ.pə.nɪm/

a word whose meaning is included in the meaning of another word:

"consolidation" is a hyponym of "lung"

"heart" is a hyponym of "mediastinum"

"steatosis" is a hyponym of "liver"