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Eye allergies, called allergic conjunctivitis, are a common condition that occurs when the eyes react to something that irritates them (called an allergen). The eyes produce a substance called histamine to fight off the allergen. As a result, the eyelids and conjunctiva — the thin, filmy membrane that covers the inside of your eyelids and the white part of your eye (sclera) — become red, swollen and itchy, with tearing and burning. Unlike bacterial or viral conjunctivitis, allergic conjunctivitis is not spread from person to person.
Reference: What Are Eye Allergies?

Amblyopia is poor vision in an eye that did not develop normal sight during early childhood. It is sometimes called "lazy eye." When one eye develops good vision while the other does not, the eye with the poorer vision is called amblyopic. Usually, only one eye is affected by amblyopia, but it is possible for both eyes to be "lazy." This condition is called bilateral amblyopia. The condition is common; approximately two or three out of every 100 people has amblyopia. The best time to correct amblyopia is during infancy or early childhood.
Reference: Amblyopia: What Is Lazy Eye?

Astigmatism is an imperfection in the curvature of your cornea — the clear, round dome covering the eye's iris and pupil — or in the shape of the eye's lens. Normally, the cornea and lens are smooth and curved equally in all directions, helping to focus light rays sharply onto the retina at the back of your eye. However, if your cornea or lens isn't smooth and evenly curved, light rays aren't refracted properly. This is called a refractive error.
Reference: What Is Astigmatism?

Blepharitis is a common and ongoing condition where the eyelids become inflamed (swollen), with oily particles and bacteria coating the eyelid margin near the base of the eyelashes. This annoying condition causes irritation, itchiness, redness, and stinging or burning of the eyes.
Reference: What Is Blepharitis?

Cataract is a clouding of the eye's lens. When we look at something, light rays travel into our eye through the pupil and are focused through the lens onto the retina, a layer of light-sensitive cells at the back of the eye. The lens must be clear in order to focus light properly onto the retina. If the lens has become cloudy, this is called a cataract.
Reference: What Are Cataracts?

A chalazion and a stye are both lumps in or along the edge of an eyelid. In some situations it may be difficult to distinguish between a chalazion and stye. A stye often appears as a red, sore lump near the edge of the eyelid, caused by an infected eyelash follicle. When a stye occurs inside or under the eyelid, it is called an internal hordeolum (pronounced "hor-dee-OH-lum").
Reference: What Are Chalazia and Styes?

Conjunctivitis is the term used to describe swelling (inflammation) of the conjunctiva — the thin, filmy membrane that covers the inside of your eyelids and the white part of your eye (known as the sclera). Often this condition is called "pink eye."
Reference: Conjunctivitis: What Is Pink Eye?

Diabetic retinopathy, the most common diabetic eye disease, occurs when blood vessels in the retina change. Sometimes these vessels swell and leak fluid or even close off completely. In other cases, abnormal new blood vessels grow on the surface of the retina.
Reference: What Is Diabetic Retinopathy?

When you blink, a film of tears spreads over the eye, making the surface of the eye smooth and clear. Without this tear film, good vision would not be possible. Sometimes people don't produce enough tears or the right quality of tears to keep their eyes healthy and comfortable. This condition is known as dry eye.
Reference: What Is Dry Eye?

You may sometimes see small specks or clouds moving in your field of vision. These are called floaters. You can often see them when looking at a plain background, like a blank wall or blue sky. Floaters are actually tiny clumps of cells or material inside the vitreous, the clear, gel-like fluid that fills the inside of your eye.
Reference: What Are Floaters and Flashes?

Glaucoma is a disease that damages your eye's optic nerve. It usually happens when fluid builds up in the front part of your eye. That extra fluid increases the pressure in your eye, damaging the optic nerve. It is estimated that three million Americans have glaucoma, but only about half of them know that they have glaucoma. Glaucoma is a leading cause of blindness for people over 60 years old. But blindness from glaucoma can often be prevented with early treatment.
Reference: What Is Glaucoma?

Hyperopia (farsightedness), is a refractive error, which means that the eye does not bend or refract light properly to a single focus to see images clearly. In hyperopia, distant objects look somewhat clear, but close objects appear more blurred. People experience hyperopia differently. Some people may not notice any problems with their vision, especially when they are young. For people with significant hyperopia, vision can be blurry for objects at any distance, near or far. It is an eye focusing disorder, not an eye disease.
Reference: Farsightedness: What Is Hyperopia?

Age-related macular degeneration (AMD) is a deterioration or breakdown of the eye's macula. The macula is a small area in the retina — the light-sensitive tissue lining the back of the eye. The macula is the part of the retina that is responsible for your central vision, allowing you to see fine details clearly.
Reference: What Is Macular Degeneration?

Myopia, or nearsightedness, is a refractive error, which means that the eye does not bend or refract light properly to a single focus to see images clearly. In myopia, close objects look clear but distant objects appear blurred. Myopia is a common condition that affects an estimated 25 percent of Americans. It is an eye focusing disorder, not an eye disease.
Reference: Nearsightedness: What Is Myopia?

Presbyopia (which literally means "aging eye") is an age-related eye condition that makes it more difficult to see very close. When you are young, the lens in your eye is soft and flexible. The lens of the eye changes its shape easily, allowing you to focus on objects both close and far away.
Reference: What Is Presbyopia?

The retina is the light-sensitive tissue lining the back of our eye. Light rays are focused onto the retina through our cornea, pupil and lens. The retina converts the light rays into impulses that travel through the optic nerve to our brain, where they are interpreted as the images we see. A healthy, intact retina is key to clear vision.
Reference: Retinal Detachment: What Is a Torn or Detached Retina?

Uveitis is inflammation of the uvea, which is made up of the iris, ciliary body and choroid. Together, these form the middle layer of the eye between the retina and the sclera (white of the eye). The eye is shaped like a tennis ball, with three different layers of tissue surrounding the central gel-filled cavity, which is called the vitreous. The innermost layer is the retina, which senses light and helps to send images to your brain. The outermost layer is the sclera, the strong white wall of the eye. The middle layer between the sclera and retina is called the uvea.
Reference: What Is Uveitis?