Nullifiers of Ablution

They are 6:
1- Whatever exits from the two private parts.
2- The exiting impure substances from other parts of the body.
3- Losing consciousness due to sleep or other than it.
4- Touching the private parts, front or back, without a barrier.
5- Eating camel's meat.
6- Leaving Islam.

Important Point:
As for washing a dead body: The correct opinion is that it does not nullify the ablution. This is the opinion of most of the scholars due to the lack of evidence to prove that. However, if the hand of the one who is washing the dead body accidentally touches the private part of the dead person without a barrier, it is a must to make ablution. Likewise, it is a must not to touch the private parts of the dead person except behind a barrier.
Moreover, touching a woman does not nullify the ablution whether it is with desire or without desire according to the correct opinion; as long as nothing comes out of his private part. This is since the Prophet صلى الله عليه وسلم kissed some of his wives and prayed without making ablution. As for the saying of Allah: "or you have touched women" it means: had relations with them according to the correct opinion which is also the opinion of Ibn Abas –may Allah be pleased with him- and other early and later scholars.