وفي الشرع: عبارة عما هو زيادة على الفرائض والواجبات
Technically, nafl is an act of worship beyond the fard and wajib requirements.
An additional prayer undertaken voluntarily, for example, seeks closeness to Allah but does not thereby become fard or wajib.
When distinguishing nafl from an emphasized Sunnah, regular observance and the nature of the religious demand are important. The terms nafl, tatawwuʿ, and mustahabb can overlap in some usages, but the present classification assigns Sunnah a distinct treatment that should not be erased.