Immensity is hugeness. When something is so large or numerous you can hardly believe it, it has the quality of immensity..
Something enormous is immense, and the quality of being immense is immensity. This is a strong word for truly gigantic things and creatures. You might, for example, comment on the immensity of the Grand Canyon, or the immensity of the night sky as seen from a boat on the sea. The noun immensity comes from the Latin word immensitatem, "immeasurableness." In other words, if it's too big to be measured, it has the quality of immensity.