'hematocrit' originates from New Latin/Greek, specifically from Greek 'haima' meaning 'blood' and from Greek root 'krinein' meaning 'to separate'.

'hematocrit' entered English in scientific/medical usage in the late 19th century (often spelled 'haematocrit' in British English). The element 'haema-' comes from Greek 'haima' and the suffix derives from Greek 'krinein' (to separate); over time the spelling without the first 'a' ('hematocrit') became common in American English.

Initially it referred to the process or practice of separating blood components; over time it came to denote specifically the measured proportion (or the measured value) of red blood cells in blood.