From MayoClinic.com

Nearsightedness or myopia, occurs when light entering the eye focuses in front of the retina instead of directly on it.
Nearsightedness (myopia) is a common vision condition in which you can see objects near to you clearly, but objects farther away are blurry. If you're nearsighted, your eye is probably too long from front to back, causing light rays to be focused in front of rather than on the back of your eyes.

The degree of your nearsightedness determines your focusing ability. People who are severely nearsighted are able to see clearly only objects that are a few inches away from their eyes. If you're mildly nearsighted, you may be able to see clearly objects that are several yards away.

Nearsightedness may develop rapidly or slowly, often gradually worsening during childhood and adolescence. The condition affects boys and girls equally, often stabilizing during young adulthood.

A basic eye exam can confirm nearsightedness. You can easily correct the condition with eyeglasses or contact lenses. Another treatment option is surgery.

Signs and symptoms 

Being nearsighted may mean:

• Distant objects appear blurry.

• You need to squint to see clearly.

• You have headaches caused by excessive eyestrain.

• Headache

Nearsightedness is often first detected during childhood, from early school years through the later teens. A child with nearsightedness may:

• Persistently squint

• Sit very close to the television, movie screen or chalkboard

• Hold books very close while reading

• Seem to be unaware of distant objects