Squint
Squint is a condition in which the eyes are not properly aligned, ie, one eye is straight and the other eye is turned away from the straight position. A horizontal squint is convergent when the deviated or the abnormal eye is turned inwards and divergent when it is turned outwards. A vertical squint is one when the abnormal eye is turned up or down. Squints can occur at any age. They can be constant, that is, present all day, or can be intermittent and therefore, occur at different times during the day; especially when the child is tired and unwell.

Binocular Vision:
It is only when we have both eyes straight, we can use them together as a pair and see things in three dimension. This is called binocular vision.

Amblyopia (Lazy Eye):
When a child has squint, he uses only his straight or normal eye to see. Vision in this eye develops normally. The squinting or deviated eye is not used. When a child has constant squint, constant disuse of the squinting eye causes decreased vision in that eye. Hence, the squinting eye becomes lazy. This is called Amblyopia. Amblyopia, is therefore, defective vision in an eye that is not uses constantly. 
Symptoms & Treatment
Symptoms
1) Babies and young children cannot say if anything is wrong with their eyes and hence, appear symptomless.

2) Older children may complain of double vision (diplopia) after the onset of squint.

3) Some children with squint can adopt an abnormal head posture, while some others tend to close one eye while under the sun or under bright light.
Treatment
Amblyopia is potentially fully correctable by appropriate treatment. The success of treatment is mainly dependent on the age of the child and the degree of amblyopia. The younger the child, the easier and quicker the recovery. Amblyopia treatment is usually not effective after the age of 10 years. There are different methods of treating amblyopia. The opthalmologist is the best person to guide about this.