According to Qur’an and Sunnah there are basically six criteria for observing hijab:
The first criterion is the extent of the body that should be covered. This is different for men and women. The extent of covering obligatory on the male is to cover the body at least from the navel to the knees. For women, the extent of covering obligatory is to cover the complete body except the face and the hands upto the wrist. If they wish to, they can cover even these parts of the body. Some scholars of Islam insist that the face and the hands are part of the obligatory extent of ‘hijab’. All the remaining five criteria are the same for men and women.
2. The clothes worn should be loose and should not reveal the figure.
3. The clothes worn should not be transparent such that one can see through them.
4. The clothes worn should not be so glamorous as to attract the opposite sex.
5. The clothes worn should not resemble that of the opposite sex.
6. The clothes worn should not resemble that of the unbelievers i.e. they should not wear clothes that are specifically identities or symbols of the unbelievers’ religions.
Complete ‘hijab’, besides the six criteria of clothing, also includes the moral conduct, behaviour, attitude and intention of the individual. A person only fulfilling the criteria of ‘hijab’ of the clothes is observing ‘hijab’ in a limited sense. ‘Hijab’ of the clothes should be accompanied by ‘hijab’ of the eyes, ‘hijab’ of the heart, ‘hijab’ of thought and ‘hijab’ of intention. It also includes the way a person walks, the way a person talks, the way he behaves, etc. (Dr. Zakir Naik)