The term 'Great Blue Expanse' is a poetic phrase used to describe the vastness of the OCEAN. The crossword solution for this term is 'OCEAN', which is a five-letter word. The OCEAN is an immense body of salt water that covers more than 70% of the Earth's surface and is home to a wide variety of marine life. It is also an important source of food, energy, and transportation for many people around the world.