The Spanish word 'lunar' comes from the Latin word 'lunaris', meaning 'relating to the moon'. 'Lunaris' itself was derived from the Latin word 'luna', which simply meant 'moon'. In Spanish, the word evolved to specifically refer to a moon-shaped mark or birthmark on the skin, likely due to the resemblance of these marks to the shape of the moon.

Several common Spanish words share this lunar connection. The most basic is 'luna' (moon), which comes directly from Latin. Other related words include 'lunes' (Monday - literally 'moon's day'), and 'lunático' (lunatic - historically believed to be affected by the moon).

English has many words derived from the same Latin root 'luna'. The most obvious is 'lunar' itself, though in English it retained its original meaning of 'relating to the moon' rather than developing the Spanish meaning of a birthmark. Other common English words include 'lunatic', 'lunacy', and 'interlunar' (between moons). The word 'month' historically tracked one complete lunar cycle, showing how the moon influenced our measurement of time.