astigmatism: An optical defect, whereby vision is blurred due to the inability of the optics of the eye to focus a point object into a sharp focused image on the retina.

blindness: There is no set definition for blindness. Usually, it is either a total loss of vision, or when there is no possibility of correcting vision through medical intervention. In Australia legal blindness is defined as best corrected visual acuity of 6/60 or below in the better eye. 

cataract: A mostly degenerative condition in which the lens of the eye clouds over, obstructing the passage of light to cause vision loss and, potentially, blindness. Cataract surgery involves the removal of the lens, replacing it with a plastic one. 

diabetic retinopathy: A disorder of the retinal blood vessels (such as microaneurysms or haemorrhaging), most often found in persons with long-standing diabetes. 

hyperopia: The inability of the eye to focus on nearby objects due to the rays of light entering the eye being brought to focus behind the retina. Also called long-sightedness or far-sightedness. 

macular degeneration: A progressive deterioration of the macula of the retina (the central inner-lining of the eye). It is often positively related to old age (usually referred to as 'age-related macular degeneration'), and results in a loss of central vision. 

myopia: A type of refractive error whereby distant objects appear blurred, usually due to light rays entering the eye being focused in front of the retina. Also called short-sightedness or near-sightedness. 

ophthalmology: The branch of medicine concerned with the study of the eye and the diagnosis and treatment of disorders of the eye. 

optometry: The practice of primary eye care, including testing for visual acuity and prescribing treatments for eye disorders. 

presbyopia: An age-related condition in which the loss of elasticity of the lens leads to a shift to long-sightedness. 

refractive error: The inability of the lens of the eye to focus on an image correctly, such as occurs in far- and near-sightedness. 

visual acuity: The ability to see clearly and at a distance. It is measured using an eye chart, and is usually expressed as a fraction. For example, 6/60 describes the ability to see objects at 6 metres that the normal eye sees at 60 metres. 

visual impairment: A significant limitation of visual capacity, including those with low vision or blindness, usually brought about by degenerative or congenital disorders, or the result of disease or injury.