The term 'Giant Heater in the Sky' is a metaphor for the SUN. The crossword solution for this term is SUN, which is a three-letter word. The SUN is a star located at the center of our solar system and is the source of light and heat for the planets that orbit it. It is composed mainly of hydrogen and helium and is the largest object in the solar system. Its immense gravity holds the planets in their orbits and its energy drives the climate and weather on Earth.