Hijab is a word that indicates not just the headscarf but clothing in its entirety which meets the following conditions (based on Quranic) :
- Clothing must cover the entire body, only the hands and face may remain visible (According to some Fiqh Schools) .
- The material must not be so thin that one can see through it.
- The clothing must hang loose so that the shape / form of the body is not apparent.
- The female clothing must not resemble the man's clothing.
- The design of the clothing must not resemble the clothing of the non believing women.
- The design must not consist of bold designs which attract attention.
- Clothing should not be worn for the sole purpose of gaining reputation or increasing one's status in society.
The reason for this strictness is so that the woman is protected from the lustful gaze of men. She should not attract attention to herself in any way. It is permissible for a man to catch the eye of a woman , however it is haram (unlawful) for a man to look twice as this encourages lustful thoughts.
Islam protects the woman. It is for this reason that Allah gave these laws.
illustration from icafegbless.com