The term 'solid part of the Earth's surface' can be summarized by two crossword solutions: LAND (4 letters) and GROUND (6 letters). LAND is a general term used to describe any solid area of the Earth's surface, including mountains, valleys, plains, and deserts. GROUND is a more specific term that refers to the soil or dirt found on the Earth's surface. Both terms are used to describe the solid parts of the Earth's surface, but LAND is a broader term that includes GROUND.