Bidah is an Arabic term used in Islam to refer to innovation in religious matters. The term means "innovation, novelty, heretical doctrine, heresy". In Islamic law, when used without qualification, bidah denotes any newly invented matter that is without precedent and is in opposition to the Quran and Sunnah. According to Shia Islam, bidah is anything that is introduced to Islam as either being fard (mandatory), mustahabb (recommended), halal (permissible), makruh (reprehensible) or haram (forbidden) that contradicts the Qurʼan or hadith. Any new good practice introduced that does not contradict the Qurʼan or hadith is permissible.

Bidah is classified into five categories of Muslim law as follows:

• Among bidʿahs required of the Muslim community (farḍ kifāyah) are the study of Arabic grammar and philology as tools for the proper understanding of the Qurʾān, evaluation of Hadith (traditions or sayings of the Prophet Muhammad) to determine their validity, the refutation of heretics, and the codification of law.
• Strictly forbidden (muḥarramah) are bidʿahs that undermine the principles of orthodoxy and thus constitute unbelief (kufr).
• Recommended (mandūb) is the founding of schools and religious houses.
• Disapproved (makrūh) are the ornamentation of mosques and the decoration of the Qurʾān.
• The law is indifferent (mubāḥah) to the bidʿahs of fine clothing and good.

It is important to note that differing from the way of the Prophet (PBUH) and falling into bidah may mean one of two things:

• Al-bidah al-haqeeqah: It means a real innovation, bidah. It is the introduction of an act of worship for which there is no foundation in Islam, such as touching graves and seeking help from their occupants, or taking the birthday of the Prophet (PBUH) as an Eid (festival). This type of bidah has no origin in Islam, and there is no basis for it within the Quran or the Sunnah, in any form whatsoever.
• Al-bidah al-idaafiyyah: This is the innovation, bidah, by addition. In this type of bidah, there is some basis for the particular act of worship, so the act itself may be found within the Sunnah, however, the innovation arises in the way the act is done.

To avoid bidah, one should refer to the Quran and Sunnah of the Prophet (PBUH) .