Li'an

Li'an - Literally Li’an means curing, legally it means an oath by which a husband claims that his wife has committed illegal sexual intercourse. The wife in her turn denies the accusation and takes a similar oath. In both oaths Allah is invoked to curse the liar. The case ends with divorce but no punishment is imposed on any of the two because there are not witnesses to confirm the accusation.

Here's a longer definition:

Li'an - 'The word is derived from the Arabic word La'n which means curslng. This takes place when a man accuses his wife of committing adultery without having four witnesses to support his claim. If he speaks publicly about it he will be considered as an accuser and will be given eighty lashes. which is tho punishment for accusation of adultery, When this incident took place during the lifetime of the Prophet the verse of Li'an was revealed (24: 6·9), which states that a husband should swear four times (in front of the magistrate and in the presence of his wife) that he is telling the truth and the fifth time he says that may the Curse of Allah be upon him if he is lying. And then she swears four times that he is lying about what he his charged her with and the fifth time she says that may the wrath of Allah be upon her if he is telling the truth. In this way he is no longer liable to be punished for false accusation and she is no longer liable to be punishment lor adultery. By this, she is eternally prohibited for him and the child she is expecting (if any) is not his.