What's the difference between 下巴 and 頷?
For modern Chinese, 下巴 is more commonly used in the daily life for the meaning of jaw or chin. Whereas 頜 is often used in medical words, like 下頜骨.

In Ancient Chinese, 頜 is more common.


  頷、頤,頜也。南楚謂之頷,秦晉謂之頜。頤,其通語也。――《方言》


ref: http://baike.baidu.com/view/269020.htm