'arroz' originates from Arabic, specifically the word 'ar-ruzz', where 'ar-' is the definite article and 'ruzz' meant 'rice'.

'ar-ruzz' passed into Old Spanish (appearing as forms like 'arroç'/'arròs') and eventually became the modern Spanish word 'arroz'.

Initially it referred to the same cereal, and over time the meaning has remained essentially 'rice' (both the grain and the cooked food).