Nasal septum deviation: Failure of the nasal septum to be in the midline where it is supposed to be. (The nasal septum is the wall inside the nose that runs down the middle dividing it into two sides.)
Deviation of the nasal septum may be congenital (present at birth) or occur later from trauma. The major problem that it tends to cause is airway obstruction. Correction is by surgery.
The "deviation" of the nasal septum is from its normal position. It is a deviant septum.