The Spanish word 'cantidad' (meaning 'quantity') comes from the Latin word 'quantitas', which also meant 'quantity'. The Latin 'quantitas' was formed by combining two elements: the interrogative adjective 'quantus' meaning 'how much' and the suffix '-itas' which was used to form abstract nouns indicating a quality or condition. Interestingly, the Latin term 'quantitas' was actually created as a calque (word-for-word translation) of the Greek word 'posótes' (ποσότης), which also meant 'quantity'.

The development from Latin 'quantitas' to Spanish 'cantidad' followed regular sound changes in the evolution from Latin to Spanish, where the 'qu' sound was simplified to 'c' and the ending '-itas' evolved into '-idad', a common pattern seen in many Spanish words derived from Latin words ending in '-itas'.

Some related Spanish words that might be familiar to Spanish learners include 'cuanto' (meaning 'how much' or 'how many'), which comes from the same Latin root 'quantus'. You can see how both 'cantidad' and 'cuanto' relate to the concept of measuring or quantifying amounts.

English speakers might recognize the connection to words like 'quantity', 'quantum', and 'quantify', which all come from the same Latin root 'quantus'. The word 'quantity' is particularly close in both meaning and form to Spanish 'cantidad', as both descended from Latin 'quantitas'. Even the scientific term 'quantum' (as in quantum physics) relates to this family, referring to a discrete amount or portion of something.