«الفرض: هو لغة: التقدير، وشرعاً: ما ثبت بدليل قطعي لا شبهة فيه»
Lexically, fard refers to fixing or determining a measure; technically it is a duty established through conclusive evidence without relevant uncertainty.
Here a conclusive proof is one that establishes the required ruling decisively. Merely finding a text is not enough to assign every detailed ruling the label fard; the authority of the proof and the clarity of its legal indication both matter.
The author uses fard in contrast with wajib, so the two definitions should be considered together.