1. a lack of something necessary for completeness or perfection; shortcoming; deficiency
2. an imperfection, failing, or blemish
3. (Chemistry) crystallog a local deviation from regularity in the crystal lattice of a solid. See also point defect, dislocation3
vb
(intr) to desert one's country, cause, allegiance, etc, esp in order to join the opposing forces
your dad deaf in one ear. If you are becoming forgetful, you can joke that you are developing a mental defect, but if it continues, you'll probably want to have your head examined.