This link explains what density refers to:
http://www.archdaily.com/180555/vide...sity-benefits/
.... urban development and smart growth is efficient space planning, not 'human stacking'. The ignorant bunch of the pro-suburban crowd generally believe that urban development means high-density apartment complexes and slum living; in reality, density includes not only high-density development, but low-density development.
It includes so much more than 'human stacking' and I wish people would understand that.