The Spanish word 'luna' comes directly from the Latin word 'luna', meaning 'moon'. The Latin word itself evolved from Old Latin 'losna', also meaning 'moon'. This is one of the most straightforward etymologies in Spanish, as the word has maintained both its form and meaning consistently from ancient times to the present day.

Many common Spanish words are derived from 'luna', including: 'lunar' (meaning 'relating to the moon'), 'lunático' (meaning 'lunatic', originally referring to someone affected by the moon's phases), and 'media luna' (meaning 'half moon' or 'crescent').

English speakers will find many familiar words that share the same Latin root 'luna'. These include 'lunar' (as in lunar eclipse), 'lunatic' (originally referring to mental illness supposedly influenced by the moon's phases), and 'lunation' (the time between one new moon and the next). The word 'loony' is also derived from this same root, being a shortened form of 'lunatic'.