'infusorian' originates from New Latin, specifically the word 'infusorium', where 'infus-' meant 'poured in' (from Latin 'infundere') and '-orium' indicated a place or instrument.

'infusorium' was used in New Latin to denote organisms found in an infusion; the English noun 'infusorian' developed from that New Latin form with the English suffix '-an' to refer to an individual member of the group.

Initially, it meant 'organisms found in an infusion (of decaying organic matter)'; over time the term remained used for such microorganisms and came to refer more generally to certain microscopic freshwater protozoa.