'certeza' originates from Latin, specifically the word 'certitās' (from the adjective 'certus'), where 'certus' meant 'settled, sure, certain'.

'certitās' passed into Vulgar Latin and then into Old Spanish/Old Portuguese as forms like 'certeza' or 'certeça', eventually becoming the modern Spanish and Portuguese word 'certeza'.

Initially, it meant 'the state of being certain' (from Latin usage), and this basic meaning has been retained into modern Spanish and Portuguese as 'certainty' or 'assurance'.