Abnormal, harmful, or undesirable effect on an organism that causes anatomical or functional damage, irreversible physical changes, or increases the susceptibility to other biological, chemical, or environmental stresses.
Optometrists can prescribe glasses, contact lenses, eye exercises, vision aids and therapy, and (in some states) oral and topical medications to treat certain eye diseases. Ophthalmologists are doctors of the eye. They have graduated from medical school and are licensed to practice medicine, so their credentials are either doctor of medicine (MD) or doctor of osteopathy (DO).
The answer comes from my understanding of these two types of eye doctors and my personal experience. Both types of eye doctors examine and prescribe glasses, diagnose and treat eye disease, and can evaluate how well a person uses the eyes together. However, each profession is unique. Ophthalmologists are trained to do surgery. I credit one with saving the eyesight of my daughter, who at age five sustained an eye injury. Optometrists are schooled in the developmental (behavioral, functional or environmental) aspects of vision.