An expanse is a huge area you can look at all at once. If you climb to the top of a mountain, suggest that your fellow climbers take time to admire the expanse of trees below.
Expanse looks like the word expand. Both come from the Latin word expandere, meaning "to spread out, unfold." But where did expandere come from? Ex- means "out" and pandere means "spread or stretch." So an expanse is something that seems to unfold before your eyes, or to put it another way, something that stretches out as far as the eye can see.