The Spanish word 'lux' comes directly from the Latin word 'lux' meaning 'light'. While the Latin word had a general meaning of light or brightness, the Spanish term has become more specialized, referring specifically to the unit of measurement for illumination in the metric system.

Some related Spanish words include 'luz' (light), 'lucir' (to shine), and 'luciérnaga' (firefly), which all share the same Latin root 'lux'. These words maintain the core meaning of light and illumination, though each has developed its own specific usage in modern Spanish.

Several English words are related to the Latin 'lux', including 'lucid' (clear, bright in thinking), 'illuminate' (to light up), and 'translucent' (allowing light to pass through). The prefix 'luc-/lum-' appears in many English words related to light, such as 'luminous', 'lucent', and 'elucidate'. Even the word 'lunar' (relating to the moon) is connected, as the moon was seen as a source of light in the night sky.