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
[C15: from Latin dēfectus, from dēficere to forsake, fail; see deficient]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.