What is a wife's nushooz, and when is this term applied to her?

A wife's nushuz (disobedience/rebellion) is her insubordination to her husband and her failure to obey him in what is obligatory upon her. This applies to any action committed by the wife as an act of disobedience to her husband, unless she was commanded to do it by the Sharia.

The remedy for nushuz is, first, admonition, then abandonment in bed, and if that does not work, then a non-brutal beating, adhering to the Sharia guidelines, as Allah Almighty says: (As for those [wives] from whom you fear nushuz, admonish them, and abandon them in their beds, and strike them.)