Squint   

Remember, when you were a kid, how you had harassed and teased the new boy, whose eyes crossed? you never knew where exactly he was looking!

When the eyes do not move in a normal coordinated manner, it results in a 'crossed eye' or Squint.

It means some part of the eye movement machinery is defective. The defect may be in the inner eye, or in the muscle, or in the nerve, or in the brain.

Though we have two eyes we see one image! Have you ever thought of thanking God for that? This miraculous gift is due to complex processing in the brain. It is called as 'Binocular vision'.

We have an innate desire to see one image. If there is a squint, two eyes will see different images. This leads to 'diplopia' or double vision. But if squint occurs in a child, the brain learns to avoid 'seeing' one of the images and we get a 'lazy eye'. The lazy eye may lose its ability to see, called as 'amblyopia'.

Ever observed your friend's eyes, when he is watching a train going in opposite direction? His eyes are just bobbing to and fro! The Medicos have a bulky name for that - Nystagmus 

Nystagmus 

Unintentional jittery/ Pendular movement of eyes is called as 'nystagmus'. It can be due to abnormality in the brain and nerves or due to abnormality in the eyes. It occurs normally when you watch a train moving in an opposite direction. It can occur with some drugs. But if seen in a baby, a consult is advisable.