The Spanish word 'solárium' comes directly from the Latin word 'solarium', which meant 'a place exposed to the sun' or 'terrace'. This Latin word was derived from 'sol' meaning 'sun', making it literally a place dedicated to sunlight. The word maintained its meaning and spelling remarkably well as it traveled through time, with only the addition of an accent mark in Spanish.

Some related Spanish words that might be familiar to Spanish learners include 'sol' (sun), 'solar' (solar), and 'soleado' (sunny). All these words share the same Latin root 'sol' and relate to concepts of sun and sunlight.

The English word 'solarium' is a direct cognate, having the same Latin origin and meaning essentially the same thing - a room designed for sunbathing or enjoying sunlight. Other related English words include 'solar', 'solstice' (from Latin 'solstitium', meaning 'sun standing still'), and 'parasol' (meaning 'protection from sun').