Incomplete or arrested development of an organ or part
The motions of the mandible are chewing, talking, yawning etc. Unwanted movement cause problems like dislocation at the hinge , trouble swallowing, choking, poor enunciation in speech , cross-bite or...
They are the bottom teeth held in place by the mandible, or lower jaw.
It is the underdevelopment of the lungs in a fetus
Short answer is, a reduced amount of blood forming tissue.