'tristylous' originates from New Latin and Ancient Greek, specifically the Greek elements 'tri-' and 'stylos', where 'tri-' meant 'three' and 'stylos' meant 'pillar' or 'style'.

'tristylous' was formed in New Latin/ botanical Latin from the Greek components (compare New Latin 'tristylus') and was adopted into modern English usage in botanical descriptions without major phonological change.

Initially, it meant 'having three styles (in flowers)', and over time this botanical meaning has remained essentially the same in modern usage.