The crossword solution for the term EARTH, IN COMBINATIONS is GEO, which is a three letter word. GEO is derived from the Greek word 'ge' meaning 'EARTH'. It is used in many combinations such as geography, geology, and geothermal. Geography is the study of the physical features of the EARTH, geology is the study of the structure and composition of the EARTH, and geothermal is the use of the EARTH's internal heat for energy.