The Spanish word 'kilo' is a shortened form of 'kilogramo' (kilogram), which ultimately comes from the Ancient Greek word 'χίλιοι' (chilioi) meaning 'thousand'. This Greek root was used to create the metric prefix 'kilo-' meaning 'one thousand'. When 'kilo' is used alone in Spanish, it typically refers to a kilogram, or one thousand grams.

Some related Spanish words that share this etymology include 'kilogramo' (kilogram), 'kilómetro' (kilometer, meaning thousand meters), and 'kilovatio' (kilowatt). All these words use the same 'kilo-' prefix to indicate one thousand of something.

English speakers will recognize many cognates using the 'kilo-' prefix, such as 'kilogram', 'kilometer', 'kilowatt', and 'kilobyte'. The prefix 'kilo-' is used identically in both languages to mean 'one thousand', making it easy to remember. Even the informal shortening 'kilo' is used similarly in English, especially when referring to kilograms.