'taxis' (as the plural of 'taxi') originates from French 'taxi', an abbreviation of 'taxicab' (from 'taximeter-cab'), where 'taximeter' was formed in the 19th century from German/French elements related to charging fees (cf. French 'taxer' 'to charge'). 'Taxis' (biology) originates from Greek 'taxis', meaning 'arrangement' or 'order'.

'Taxi' developed in the late 19th century as a shortening of 'taxicab' (from 'taximeter-cab'), and its plural became 'taxis' in English. The biological term comes from Greek 'taxis', passed into New Latin for scientific use and was adopted into modern English biological terminology in the 19th century.

For the vehicle sense, the origin relates to charging/measurement (taximeter) and shifted to name the hired vehicle; the plural 'taxis' simply denotes multiple such vehicles. The Greek 'taxis' initially meant 'arrangement' or 'order' and evolved in biology to mean 'directed movement' in response to stimuli.