See below. The nasal septum is the bone and cartilage partition which divides the nose into two sides. When it is crooked and not in the middle of the nasal cavity, it is deviating to one side or the other. The condition is corrected with a surgical procedure called septoplasty. In this surgery, the crooked bone and cartilage are reshpaed or removed to allow the septum to be positioned in the midline.
Deviated = displaced. A deviated septum is one that is out of alignment. The septum of the nose should be down the middle to allow air to pass equally on the right and left sides. When it is deviated the air passage will be blocked, usually one side of the other. The correction is surgical in that the septum has to be mechanically moved and manipulated to be in the middle again.