虐待 (nüè dài) means 'to mistreat, to abuse' – it refers to cruel, often systematic infliction of harm (physical or psychological), as in child or animal abuse. It's stronger and more formal than 亏待 (kuī dài 'to treat unfairly'), which is about unfairness rather than brutality. 折磨 (zhé mó 'to torment') focuses more on causing prolonged suffering, but 虐待 emphasizes the act of mistreatment itself.