Niqaab is that with which a woman veils her face (tantaqib)… 
“And tell the believing women to lower their gaze (from looking at forbidden things), and protect their private parts (from illegal sexual acts) and not to show off their adornment except only that which is apparent (like both eyes for necessity to see the way, or outer palms of hands or one eye or dress like veil, gloves, headcover, apron), and to draw their veils all over Juyoobihinna (i.e. their bodies, faces, necks and bosoms)…”

[al-Noor 24:31] 

Drawing the veil all over the juyoob implies covering the face. 

When Ibn ‘Abbaas (may Allaah be pleased with him) was asked about the aayah (interpretation of the meaning): 

“O Prophet! Tell your wives and your daughters and the women of the believers to draw their cloaks (veils) all over their bodies”

[al-Ahzaab 33:59] – 

A woman is obliged to cover her face in front of anyone who is not her mahram (blood relative to whom marriage is forbidden). From this we learn that the face is the most essential thing to be covered. There is evidence from the Book of Allaah and the Sunnah of His Prophet (peace and blessings of Allaah be upon him) and the views of the Sahaabah and the imams and scholars of Islam, which indicates that women are obliged to cover all of their bodies in front of those who are not their mahrams.