According to Reference.com, the term intercostal is defined as "between the ribs." Intercostal refers to muscles, veins or arteries between the ribs. There are several groups of muscles that run between the ribs. They serve an important function by helping the chest wall move.
There are three layers that make up the intercostal muscles. The two most important layers are the external intercostal muscles and the internal intercostal muscles. The external intercostal muscles aid in quiet and forced inhalation. The internal intercostal muscles aid in forced expiration. They also cause the rib cage to depress during exhalation. Both of these muscle layers are innervated by the intercostal nerves and are serviced by the intercostal arteries and intercostal veins.