The nadhr is a self imposed vow, distinct in Islamic law from the five daily obligatory prayers and the other fixed pillars precisely because nothing compels a person to make one, yet once made, classical jurisprudence treats it as binding, grounded in Quranic verses that command believers to fulfil their vows and in hadith reports warning against making a vow lightly while still requiring its fulfilment once undertaken. A nadhr is commonly conditional, promising an act of worship, charity or fasting if a specific hoped for outcome occurs, a sick child's recovery, a safe journey, success in an undertaking, though an unconditional vow made purely as an act of devotion is equally recognised.
Jurists distinguish a valid nadhr, which must commit to something already lawful and generally pleasing to God, from an invalid one, such as a vow to commit a sin or to do something already religiously obligatory, which classical scholars hold cannot properly be the object of a vow at all since a person owes it regardless. Where circumstances make fulfilment of a specific vow impossible, Islamic law provides for kaffara, an expiation typically involving feeding or clothing a set number of poor people, fasting, or freeing a slave in the classical formulation, mirroring the expiation prescribed for a broken oath more generally.