SALATU L-WITR. صلاة الوتر
The Witr prayers. The word witr means either a unit, or an odd number and as used for either a single or odd number of rack’ah prayers recited after the evening prayer (‘isha). Mishkat, book iv. Ch. xxxvi.)
There is considerable controversy amongst the learned doctors as to whether it is farz, wajib, of sunnah, but it is generally held to be sunnah, i.e. founded on the example of the Prophet, but with no divine command. Amongst the Hanafi sect, it is also known as Qunutu ‘l-Witr, but the Shafi’is recite the Qunut separately.

Based on Hughes, Dictionary of Islam