The Spanish word 'termo' (meaning 'thermos' or 'vacuum flask') comes from the English brand name 'Thermos', which in turn derives from the Ancient Greek word 'θερμός' (thermós) meaning 'hot' or 'warm'. This is a case where a brand name became so widely used that it became the generic term for the product in Spanish, similar to how 'Kleenex' is often used for any tissue in English. The Thermos company, founded in 1904, derived its name from the Greek word because their vacuum flasks were designed to keep hot liquids hot (and cold liquids cold).

Several common Spanish words share the same Greek root 'θερμός', including: 'térmico' (thermal), 'termómetro' (thermometer), and 'termal' (thermal, relating to heat). These words all relate to the concept of heat or temperature, maintaining a clear connection to the original Greek meaning.

Many English words share this Greek root 'θερμός', making them cognates with 'termo'. Some common examples include 'thermal' (relating to heat), 'thermometer' (temperature measuring device), 'thermostat' (temperature control device), and of course 'thermos' itself. The prefix 'thermo-' is very productive in English, appearing in many scientific and technical terms related to heat and temperature.