The abaya is a modest dress that is worn by many Muslim women especially who are living in Middle East countries. Abayas come in many different designs, fabrics and colors that serve the purpose of modest clothing for Islamic people.
There is no definitive ruling in Islam stating that women must wear the abaya. The Quran instructs both men and women to dress and behave modestly. The exact interpretation of what modest clothing entails varies between cultures, traditions and scholarly opinions. The key requirements are that clothing should cover the parts of the body that are considered awrah (intimate parts) and that it should be loose enough so as not to reveal the figure. How this is specifically achieved varies around the Muslim world.
In some countries like Saudi Arabia and Qatar, it is obligatory by law for Muslim women to wear an abaya when in public. However, many Islamic scholars argue that these national laws do not make the abaya dress itself obligatory in Islam. Rather, wearing some type of loose and modest outer-garment is obligatory according to most interpretations.
The obligation derives from Quranic verses instructing women to “draw their cloaks over their bodies” and for both men and women to “guard their private parts”. These instructions allow for different cultural interpretations and styles of dress, with the abaya being common in Gulf countries as a garment that enables women to follow Quranic guidelines on modesty.
So while the abaya itself may not be religiously obligatory, most scholars emphasize that wearing loose, opaque and non-revealing clothing that covers everything but the hands, feet and face is. This type of modest dress is considered obligatory upon reaching maturity. Some scholars differentiate between a woman dressing this way in front of non-mahram (marriageable) men versus in front of other women or male family members. So the obligation may vary depending on the social context.
In summary, while national laws of certain Muslim countries may require wearing the abaya in public, it is not defined by Islam itself as a compulsory garment. The focus in Islamic teachings is for both men and women to dress and behave modestly. Wearing a loose over-garment that doesn’t reveal body shape is seen as obligatory by most scholars when in contexts that require modesty and chastity. But the specific style or cultural form this takes - be it an abaya, jilbab, chador, kaftan or other garment - is not uniformly obligatory. Essentially, Muslim women must cover themselves modestly, but can choose how to achieve this based on their cultural preferences and interpretations of Islamic guidelines on modest clothing.