In Islamic fiqh, Hadath-Akbar refers to a state of major ritual impurity. The condition of Hadath-Akbar requires the complete ablution (al-ghusl) before one can pray, circumambulate the Kaaba; touch, carry, or recite the Quran; or stay at the mosque. Five actions place one in the state of Hadath-Akbar, including discharge of sperm (al-mani) owing to stimulation while asleep or awake, sexual intercourse with or without ejaculation, and menstruation. The phrase "hadath-akbar" can be directly translated as "great event", since "hadath" means "event" in Arabic 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.