The term 'Expanses of H2O' refers to the vast bodies of water found on Earth, such as seas, oceans, and other large bodies of water. The crossword solution for this term is 'SEAS', which is an acronym for 'saltwater ecosystems and aquatic systems'. This term encompasses all of the different types of water found on Earth, including saltwater and freshwater, and is used to describe the large bodies of water that cover most of the planet's surface.