The Spanish word 'kelvin' (unit of temperature) comes from the title 'Baron Kelvin', which was given to the renowned physicist William Thomson. The title itself was derived from the name of the River Kelvin, a narrow river in Scotland near the University of Glasgow where Thomson worked. The river's name has Celtic origins, where it meant 'narrow river'.

William Thomson made significant contributions to thermodynamics and other areas of physics, and when he was elevated to the peerage in 1892, he chose his title 'Baron Kelvin' after this river. The temperature unit was later named in his honor, and this term has been adopted into Spanish and many other languages.

Since 'kelvin' entered Spanish as a direct borrowing of a modern scientific term, there aren't any simpler or more common Spanish words that share its etymology.

The English word 'kelvin' is identical to the Spanish, as both languages borrowed this scientific unit directly from the title of Baron Kelvin. In English, you might also see 'K' used as its symbol in scientific contexts, as in '373K' (100°C).