The hijab per se is not a requirement. It is, however, a type of veil (حَجَّبَ) so it does qualify in meeting the requirement of not displaying the “adornment” and covering “bosom”. Which is the requirement.
Qur’an 24:31 And say to the female believers to cast down their be holdings, and preserve their private parts, and not display their adornment except such as is outward, and let them fix (Literally: strike)closely their veils over their bosoms, and not display their adornment except to their husbands, or their fathers, or their husbands’ fathers, or their sons, or their husbands’ sons, or their brothers, or their brothers’s sons, or their sisters’ sons, or their women, or what their right hands possess, or (male) followers, men without desire (Literally: without being endowed with “sexual” desire) or young children who have not yet attained knowledge of women’s privacies, and they should not strike their legs (i.e., stamp their feet) so that whatever adornment they hide may be known. And repent to Allah altogether, (O) you believers, that possibly you would prosper.
Arabic: وقل للمؤمنات يغضضن من ابصارهن ويحفظن فروجهن ولا يبدين زينتهن الا ما ظهر منها وليضربن بخمرهن على جيوبهن ولا يبدين زينتهن الا لبعولتهن او ابائهن او اباء بعولتهن او ابنائهن او ابناء بعولتهن او اخوانهن او بني اخوانهن او بني اخواتهن او نسائهن او ما ملكت ايمانهن او التابعين غير اولي الاربة من الرجال او الطفل الذين لم يظهروا على عورات النساء ولا يضربن بارجلهن ليعلم ما يخفين من زينتهن وتوبوا الى الله جميعا ايه المؤمنون لعلكم تفلحون
The word veil (حَجَّبَ) should be thought of as a covering, concealment, and screening off and not any one specific type of attire or garment.