Density
The degree of visual saturation, sparse or crowded, within a given area of a composition.
In practice
Density describes how packed an area is, from a few elements with plenty of room to many crowded together. Designers vary it to set mood and pace: a luxury catalog page might hold one product and a short line of text, while a train timetable is dense by necessity. Contrasting dense and sparse areas within one layout also guides attention, because the eye notices the change.
Often confused with Mass
Density is how crowded an area is with elements; mass is the perceived weight of a single solid form.
Try this
Design one page holding a single word and another holding fifty words, and note how that word feels on each.