Other forms: countersuits
If you take someone to court, they may respond by filing a countersuit against you. If a tenant sues their landlord for unfairly evicting them, the landlord might file a countersuit claiming months of unpaid rent.
In law, a countersuit is sometimes called a counterclaim. It's what happens when one person sues another, who then responds by suing them. Imagine getting in a car accident that wasn't clearly the fault of either driver. If the other person is injured, she might sue you to pay her doctor bills. If you, in turn, have a hurt neck requiring a trip to the hospital, you could file a countersuit, hoping to make her pay your medical expenses.