The Experiencing of Impurity in Salaat
If a person experiences hadath in his salaat, his salaat will become invalid if he experienced hadath-e-akbar, whereby ghusl becomes wajib. If he experiences hadath-e-asghar, it will either be two things: this hadath-e-asghar will either be ikhtiyaari or ghair ikhtiyaari. Ikhtiyaari means it was caused or brought about by the will of man. Ghayr ikhtiyaari means that it was not caused or not brought about by the will of man.