Al-Maani’ linguistically: It is what prevents and is obstructive to other than it.

The meaning in accordance to the Istilaah (Terminological definition):

It is what the Shaari’ has made the absence of the Hukm resultant from its presence or the absence of the Sabab (cause) of the Hukm resultant from its existence (i.e. if it exists the Hukm does not exist or the Sabab for the Hukm does not exist).

Example:

Al-Haid (menstruation) is a Maani’ (preventer) for the woman’s Salaah.

The debt represents a Maani’ (preventer) for the attainment of the Nisaab of the Zakaah if the debt reduces the amount under the Nisaab.