Who is an expert?

This section permits only the opinion of an expert to be cited in evidence. This section requires the determination of the question as to who is an expert. The only guidance in the section is that he should, be a person especially skilled on the matter. 
In a matter of spying, the question was whether a document recovered from the accused was of strategic importance to the country. A finding was recorded after obtaining the opinion of army officials and careful scrutiny of ocular evidence and written submissions. The court said that it was not liable to be interfered with on the ground that the army officials were not experts?

Generally, a person having special knowledge and capability is considered to be an Expert, because he is specialized in that subject. Sometimes, the experience is also included in it. Thus, two things may be required for an Expert. 
(a) Special Knowledge 
(b) Experience 
A person who has obtained special knowledge or experience by hard work, examination and continuously study, that person may be called as an expert. 
It has been said by the Mysore High Court that Expert does not require any degree. 
But, the opinion of the author in books is not considered to be the opinion of Experts. No one can be convicted on the basis of such an opinion. 

Subjects on which expert can testify 
    Foreign law 
    Matter of science 
    Questions of art 
    Identity of Handwriting 
    Finger impressions

It is upon the discretion of the court to accept or not the opinion of the handwriting expert. 

Whether the opinion of the expert is binding 

The court is not bound to accept the opinion of the expert. 
The opinion of the expert is held to be weak evidence. Such evidence must be considered with extra care and caution. 
It is clearly been stated by the Supreme Court that the opinion of an expert is merely an opinion. It 

Watch this video on Expert's opinion in Indian Evidence act