وفي الشرع: ما ثبت بدليل قطعي لا شبهة فيه
Technically, fard is a ruling established by definitive evidence that is free from relevant doubt.
Definitive proof means that the proof establishes the particular ruling with certainty. Merely being widely quoted or frequently mentioned does not establish this status: the reliability of the proof and its indication of the specific ruling must be considered.
The obligation of the five daily prayers and fasting in Ramadan are examples established on a definitive basis. They are therefore not merely encouraged acts of devotion.