What are the necessary conditions to see an object ? What is light?

What are the necessary conditions to see an object ? What is light ?

Please answer fast.

main point to be noted that the object should posses the property of opaquicity i.e the body should be opaque.The second property that the light ray should reflect over the body due to which our eyes can easily see the object.
LIGHT-- It can be defined as the phenomena which makes the object to see. It is group of waves travelling. The speed of light "denoted by ā€˜Cā€™ is 3*10^8

@yoginder