A type wrapped cloth head covering.
Professor Quirrell wore a very large turban to hide Voldemort's face on the back of Quirrell's head during Harry's first year (PS17).
The night after the sorting ceremony, Harry has a dream that he is wearing Quirrell's turban and it is telling him to switch to Slytherin House (PS7).
Commentary
Notes
Turbans are worn in many cultures throughout the world. The depiction of Quirrell's turban in the chapter art for for PS17 in the American version of the book and the general description of it as very large make it sound most similar to those worn in the Ottoman Empire hundreds of years ago. Geographically, this might make sense, seeing as Albania was part of the Ottoman Empire. (source: Wikipedia) -BB
For another instance in YA literature of a turban being used by a teacher at a boarding school to hide the telltale evidence of the series' main villain, see A Series of Unfortunate Events No. 5, The Austere Academy by Lemony Snicket. -BB
Harry's dream in PS7 about Quirrell's Turban telling him he should be in Slytherin means far more than Harry could have realized then. The turban hides Voldemort, and a piece of Voldemort's soul is hidden in Harry and has some ability to affect him. -BB