'unisonal' originates from Latin, specifically the word 'unisonus', where 'unus' meant 'one' and 'sonus' meant 'sound'. The modern adjective is formed in English by adding the suffix '-al' to 'unison'.

'unisonal' changed from the Late Latin/Old French form 'unisonus' to the Middle English noun 'unison' and eventually became the Modern English adjective 'unisonal' by suffixation ('unison' + '-al').

Initially it referred to the idea of a 'single sound' (literally 'one sound'); over time it evolved into the adjective meaning 'sounding together' or 'performed in unison'.