Glaucoma is a group of diseases of the optic nerve involving loss of retinal ganglion cells in a characteristic pattern of optic neuropathy. Although raised intraocular pressure is a significant risk factor for developing glaucoma, there is no set threshold for intraocular pressure that causes glaucoma. One person may develop nerve damage at a relatively low pressure, while another person may have high eye pressure for years and yet never develop damage. Untreated glaucoma leads to permanent damage of the optic nerve and resultant visual field loss, which can progress to blindness.

Glaucoma has been nicknamed "the silent sight thief". Worldwide, it is the second leading cause of blindness. Glaucoma affects 1 in 200 people aged 50 and younger and 1 in 10 over the age of 80. Children under the age of 5 are also considered at high risk, with around 1 in 50 developing the early stages of the disease.