the hijab, or veil, is a matter of choice. 1985 Times 5 Jan. 8/3 Every woman on the street wears either the traditional chador..or the more practical hijab, a dark scarf pulled over the forehead. 1994 J. I. SMITH in A. Sharma Today's Woman in World Relig. 306 Many Egyptians who do not adopt the higab are deeply respectful of those who do. 1995 New Yorker 30 Jan. 60/3 A young girl..who was dressed in a black djellabah and wearing the traditional head scarf, or hijab. 2005 Asiana Spring 278/1 (heading) Wearing a hijab is no barrier to success."1980 Associated Press Newswire (Nexis) 15 July, She said the wearing of
Notice that only the last one makes the unmarked choice of a instead of the. This suggests that it is a symbol rather than merely a piece of clothing. Other somewhat analogous items include: veil, burka, turban, kirpan, cross, and robes of office.
Unfortunately, none of these is entirely satisfactory. For a nun, taking up the veil, entails more than simply wearing it. To a certain extent, the same could be said for hijab, but once a nun takes up the veil, it doesn't matter whether or not she is actually wearing it at a given time. As this story shows, the same is not true of hijab. Also, burka and hijab should, theoretically, pattern identically, but they don't. Although the OED has examples such as, "1929 Daily Express 15 Jan. 1/1 The Queen [of Afghanistan] is wearing the boorka--a heavy shapeless garment which effectually hides her beauty," current usage seems to favour "a burka". Here are the counts from the Google news archive:
With turban, we find 4,540 for the vs. 12,800 for a. And kirpan is almost in a dead heat.
Clearly this data is very muddy, but I think it is at least suggestive that there is something different going on with hijab.