Ḥiyal (حيل, singular ḥīla حيلة "contortion, contrivance; device, subterfuge"[1]) is "legalistic trickery" in Islamic jurisprudence.
A special sub-field of ḥiyal is "oath-trickery" (maʿārīḍ) dedicated to the formulation of ambiguous statements designed to be interpreted as an oath or promise while leaving open loopholes to avoid perjury.
The earliest development of this field is the Kitāb al-maḫārij fī l-ḥiyal ("book of evasion and trickery") by Muhammad al-Shaybani (d. 805).
Since the 15th century, Shafiite opposition to ḥiyal had mostly disappeared, due to the fatwas by Ibn Hajar al-Asqalani outlawing its criticism.
In 1974, a publication by Muhammad ʿAbd-al-Wahhāb Buhairī, Al-Azhar University professor for hadith and fiqh, published a monograph on the question Al-Ḥiyal fi š-šarīʿa al-islāmīya ("trickery in Islamic law"), according to which only a limited number of ḥiyal are permissible.