<unk> is the volume of air that is inhaled that does not take part in the gas exchange, because it either remains in the conducting airways or reaches alveoli that are not perfused or poorly perfused. It means that not all the air in each breath is available for the exchange of oxygen and carbon dioxide. Mammals breathe in and out of their lungs, wasting that part of the inhalation which remains in the conducting airways where no gas exchange can occur.
Dead space 
Granulation tissue
Enema
Contour threads
Granulation
Protein corona
Fluid compartments
Underwood's septa