In Islamic fiqh

Fiqh

Fiqh is Islamic jurisprudence. Fiqh is an expansion of the code of conduct expounded in the Quran, often supplemented by tradition and implemented by the rulings and interpretations of Islamic jurists....

, Hadath-Akbar refers to a state of major ritual impurity. The condition of Hadath-Akbar requires the complete ablution

Ghusl

Ghusl is an Arabic term referring to the full ablution required in Islam for various rituals and prayers. The ablution becomes mandatory for any adult Muslim after having sexual intercourse, any sexual discharge , completion of the menstrual cycle, giving birth, and death by natural causes.Islam...

 (al-ghusl) before one can pray, circumambulate the Kaaba

Kaaba

The Kaaba is a cuboid-shaped building in Mecca, Saudi Arabia, and is the most sacred site in Islam. The Qur'an states that the Kaaba was constructed by Abraham, or Ibraheem, in Arabic, and his son Ishmael, or Ismaeel, as said in Arabic, after he had settled in Arabia. The building has a mosque...

; touch, carry, or recite the Quran; or stay at the mosque

Mosque

A mosque is a place of worship for followers of Islam. The word is likely to have entered the English language through French , from Portuguese , from Spanish , and from Berber , ultimately originating in — . The Arabic word masjid literally means a place of prostration...

. Five actions place one in the state of Hadath-Akbar, including discharge of sperm

Semen

Semen is an organic fluid, also known as seminal fluid, that may contain spermatozoa. It is secreted by the gonads and other sexual organs of male or hermaphroditic animals and can fertilize female ova...

 (al-mani) owing to stimulation while asleep or awake, sexual intercourse with or without ejaculation

Ejaculation

Ejaculation is the ejecting of semen from the male reproductory tract, and is usually accompanied by orgasm. It is usually the final stage and natural objective of male sexual stimulation, and an essential component of natural conception. In rare cases ejaculation occurs because of prostatic disease...

, and menstruation

Menstruation

Menstruation is the shedding of the uterine lining . It occurs on a regular basis in sexually reproductive-age females of certain mammal species. This article focuses on human menstruation.-Overview:...

. The phrase "hadath-akbar" can be directly translated as "great event", since "hadath" means "event" in Arabic

Arabic language

Arabic is a name applied to the descendants of the Classical Arabic language of the 6th century AD, used most prominently in the Quran, the Islamic Holy Book...

and "akbar" means "great" in Arabic.

Passing wind also places the believer in a state of Hadath (ritually unclean), but it is considered a Hadath-Asghar (literally minor event) and only requires wudhu to purify from it.

Translation of Sahih Bukhari, Book 4:
Volume 1, Book 4, Number 176:
Narrated Abu Huraira:

Allah's Apostle said, "A person is considered in prayer as long as he is waiting for the prayer in the mosque as long as he does not do Hadath." A non-Arab man asked, "O Abii Huraira! What is Hadath?" I replied, "It is the passing of wind (from the anus) (that is one of the types of Hadath)."

The complete ablution requires that one wash the entire body with water.

Hadath Akbar is part of Najassah Hokmi, this mean that there are many "if this & if that's". However it is overall quite basic with rulings like Najas Ghaleeza.