‘Iṣmah or ‘Isma (Arabic: عِصْمَة; literally, "protection") is the concept of incorruptible innocence, immunity from sin, or moral infallibility in Islamic theology, and which is especially prominent in Shia Islam. In Shia theology, ismah is characteristic of prophets, imams, and angels. When attributed to human beings, ismah means "the ability of avoiding acts of disobedience, in spite of having the power to commit them". Along with a pure constitution, excellent qualities, firmness against opponents, and tranquility (as-Sakinah), ismah is a divine grace bestowed by God.

An infallible (Arabic: معصوم, romanized: ma'sūm) is someone who is free from error in leading people to belief, in perceiving divine knowledge, and in practical matters. Prophets must be immune from all errors and sins in order to perform their mission of upholding and promoting the divine religion, interpreting the Qur'an, and establishing a wholesome social system.

According to Twelver Shia, The Fourteen Infallibles (Arabic: معصومون, romanized: Ma‘ṣūmūn) "divinely bestowed free from error and sin" include Muhammad, his daughter Fatimah, and the Twelve Imams. Ismaili also attribute ismah to Ismaili imāms and Fatimah, daughter of Muhammad, while Zaidis do not attribute the quality to the Zaidi imams.

The doctrine of ismah has been rejected by some Muslims, such as the Kharijites who cited Quran 48:2 as evidence for the rejection.

Sunnis interpret ismah to mean that prophets are immune from telling lies (intentionally or unintentionally), of being Kafir (infidel) before or after their assignment, and of being unable to commit other sins intentionally. In other aspects, opinions diverge. Most Sunnis believe that it is possible for the prophets to unintentionally commit sin, while the minority believe that it is not.

The purity of Ahl al-Bayt, the family of Muhammad, is manifested by the Verse of Purification in the Qur'an. The development of Shi'ite theology in the period between the death of Muhammad and the disappearance of the Twelfth Imam extends this concept of purity and originates the concept of ismah. The concept of the immunity from sin (ma'sum) of the imams, the Imamiyyah, perhaps began in the first half of the second century AH. Shia scholars of the fourth and the fifth centuries AH extended the infallibility of Muḥammad and the Twelve Imams until the doctrine came to mean that they could not have committed any sin or inadvertent error either before or after they assumed office.